Citing "von lastname", "von lastname" in bibliography, but sort by "lastname"

Question

I am using \usepackage[style=authoryear-comp, useprefix=true]{biblatex}

I am aware that with the useprefix option I can turn on/off using the "von" for both citations and bibliography. However, I want to use "von lastname" in both but sort by "lastname" in the bibliography. "von" preferably in lowercase. This seems to be possible with adding sortkey = {lastname, firstname von} to each entry (like in this post: Sorting of author names like "von Beethoven" with biblatex).

But I am importing my .bib file from Zotero, so I cannot modify it manually (e.g. to add a field).

I would go so far as to add "sortkey = lastname, firstname von" in Zotero somehow, but I don't see how this would be possible. It doesn't seem to work in the Extra field.