How do I add Pseudonym after author name

Question

I was wondering if it is possible to add (Pseud.) after an author and editors name if the name is somewhat labelled as "pseudonym". What would be the best practice for labelling an author as Pseud? The idea would be to have it behave like (Eds. or Hg.) for Editors, so it would appear after an authors name in parantheses. The order (in case of Ed.) would be in front of Editors (Pseud.) (Hg.) (2000) : Title...

MWE:

\documentclass[
  12pt,
  a4paper,
  headings=standardclasses,
  listof=totoc,
  numbers=noenddot
]{scrartcl}
\usepackage[left=2.50cm, right=2.50cm, top=2.50cm, bottom=2.00cm, footskip=1cm]{geometry}
\usepackage[onehalfspacing]{setspace}
\usepackage[main=ngerman, english]{babel}
\usepackage[babel, german=quotes]{csquotes}
\usepackage[ngerman]{isodate}
\usepackage[ngerman]{datetime}
%----------------------------------------------------------------------------
%   BIB
%----------------------------------------------------------------------------
\usepackage[
  backend=biber,
  style=ext-authoryear,
  sorting=nyvt,
  datamodel=customstyles,
  maxnames=25,
  innamebeforetitle=true,
  usetranslator=true,
  alldates=terse,
  labeldate=year,
  dashed=false,
  doi=false,
  isbn=false
]{biblatex}
\addbibresource{\jobname.bib}
\usepackage[hidelinks, pdfencoding=auto]{hyperref}
\DeclareDelimFormat[bib]{nametitledelim}{\addcolon\space}
% editor in parentheses
\DeclareDelimFormat{editortypedelim}{\addspace} 
\DeclareFieldFormat{editortype}{\mkbibparens{#1}}
\DeclareDelimAlias{translatortypedelim}{editortypedelim}
\DeclareFieldAlias{translatortype}{editortype}
\DeclareFieldFormat
  [article,inbook,incollection,inproceedings,patent,online]
  {title}{#1\isdot}
%--------------------------------------------------------------------
%   MAIN
%--------------------------------------------------------------------
\begin{filecontents}{\jobname.bib}
@book{Karnath.2012,
    addendum = {(3., aktualisierte und erweiterte Auflage)},
    title = {Kognitive Neurowissenschaften},
    author = {Karnath, Hans-Otto and Thier, Peter},
    place = {Berlin, Heidelberg},
    publisher = {Springer},
    year = {2012}}
@collection{Karnath.2012b,
    addendum = {(3., aktualisierte und erweiterte Auflage)},
    title = {Kognitive Neurowissenschaften},
    editor = {Karnath, Hans-Otto and Thier, Peter},
    place = {Berlin, Heidelberg},
    publisher = {Springer},
    year = {2012}}
\end{filecontents}
\begin{document}
\parencite{Karnath.2012} \parencite{Karnath.2012b}
\clearpage
\printbibheading[heading=bibintoc,title={Literaturverzeichnis}]
\printbibliography
\end{document}

Answer 1

You can use item annotations to mark up pseudonyms (see for example also https://tex.stackexchange.com/a/304968/35864, https://tex.stackexchange.com/a/317642/35864, BibLaTeX: prevent caps for some authors, Indicating joint first authorship through special markup in biblatex/biber, Biblatex multiple author annotations). Then we need to inject code printing the pseudonym info in the name formats.

\documentclass[12pt, a4paper]{scrartcl}
\usepackage[main=ngerman, english]{babel}
\usepackage[babel, german=quotes]{csquotes}
\usepackage[
  backend=biber,
  style=ext-authoryear,
  innamebeforetitle=true,
  dashed=false,
]{biblatex}
\usepackage[hidelinks, pdfencoding=auto]{hyperref}
\DeclareDelimFormat[bib]{nametitledelim}{\addcolon\space}
\DeclareDelimFormat{editortypedelim}{\addspace} 
\DeclareFieldFormat{editortype}{\mkbibparens{#1}}
\DeclareDelimAlias{translatortypedelim}{editortypedelim}
\DeclareFieldAlias{translatortype}{editortype}
\DeclareFieldFormat{edition}{\mkbibparens{#1}}
\NewBibliographyString{pseudonym}
\DefineBibliographyStrings{german}{
  pseudonym = {Pseud\adddot},
}
\newbibmacro*{name:pseudonym}{%
  \ifitemannotation{pseudonym}
    {\addspace
     \mkbibparens{%
       \bibstring{pseudonym}}}
    {}}
\DeclareNameFormat{given-family}{%
  \ifgiveninits
    {\usebibmacro{name:given-family}
      {\namepartfamily}
      {\namepartgiveni}
      {\namepartprefix}
      {\namepartsuffix}}
    {\usebibmacro{name:given-family}
      {\namepartfamily}
      {\namepartgiven}
      {\namepartprefix}
      {\namepartsuffix}}%
  \usebibmacro{name:pseudonym}%
  \usebibmacro{name:andothers}}
\DeclareNameFormat{family-given}{%
  \ifgiveninits
    {\usebibmacro{name:family-given}
      {\namepartfamily}
      {\namepartgiveni}
      {\namepartprefix}
      {\namepartsuffix}}
    {\usebibmacro{name:family-given}
      {\namepartfamily}
      {\namepartgiven}
      {\namepartprefix}
      {\namepartsuffix}}%
  \usebibmacro{name:pseudonym}%
  \usebibmacro{name:andothers}}
\DeclareNameFormat{family-given/given-family}{%
  \ifnumequal{\value{listcount}}{1}
    {\ifgiveninits
       {\usebibmacro{name:family-given}
         {\namepartfamily}
         {\namepartgiveni}
         {\namepartprefix}
         {\namepartsuffix}}
       {\usebibmacro{name:family-given}
         {\namepartfamily}
         {\namepartgiven}
         {\namepartprefix}
         {\namepartsuffix}}%
     \ifboolexpe{%
       test {\ifdefvoid\namepartgiven}
       and
       test {\ifdefvoid\namepartprefix}}
       {}
       {\usebibmacro{name:revsdelim}}}
    {\ifgiveninits
       {\usebibmacro{name:given-family}
         {\namepartfamily}
         {\namepartgiveni}
         {\namepartprefix}
         {\namepartsuffix}}
       {\usebibmacro{name:given-family}
         {\namepartfamily}
         {\namepartgiven}
         {\namepartprefix}
         {\namepartsuffix}}}%
  \usebibmacro{name:pseudonym}%
  \usebibmacro{name:andothers}}
\DeclareFieldFormat
  [article,inbook,incollection,inproceedings,patent,online]
  {title}{#1\isdot}
\begin{filecontents}{\jobname.bib}
@book{Karnath.2012,
  title     = {Kognitive Neurowissenschaften},
  author    = {Karnath, Hans-Otto and Thier, Peter},
  author+an = {2=pseudonym},
  place     = {Berlin and Heidelberg},
  publisher = {Springer},
  year      = {2012},
  edition   = {3., aktualisierte und erweiterte Auflage},
}
@collection{Karnath.2012b,
  title     = {Kognitive Neurowissenschaften},
  editor    = {Karnath, Hans-Otto and Thier, Peter},
  editor+an = {2=pseudonym},
  place     = {Berlin and Heidelberg},
  publisher = {Springer},
  year      = {2012},
  edition   = {3., aktualisierte und erweiterte Auflage},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
Lorem \autocite{Karnath.2012,Karnath.2012b}
\printbibliography[heading=bibintoc, title={Literaturverzeichnis}]
\end{document}