I am using the solution as posted in this answer:
https://tex.stackexchange.com/a/21891/8569
The important snippet is:
\addtocategory{important}{walley00}\nocite{walley00}
\addtocategory{important}{walley91}\nocite{walley91}
\printbibliography[title={Important},category=important]
Instead, I would prefer a new command like this:
\addtocategoryNoCi{important}{walley00,walley91}
\printbibliography[title={Important},category=important]
The effect should be the same. So, I guess I just need to define a new/custom command that accepts a comma separated list of bibtex keys as input, but I need some help with that...