I solved my problem by editing the properties afterwards, so instead of committing with a fake username, which I can't make my server do, I do this :
svn propset --revprop -r HEAD svn:author NEW_AUTHOR_NAME
To make this work, you have to enable the "pre-revprop-change-hook" (example here)
BEcause somebody asked - My task is to migrate the complete history of our Code , which was until now managed by a very outdated version control system (QVCS), to Subversion. TO do this, i had to edit every user, because some of them aren't working here anymore and so i have to make my system to believe me ;)
Normally i wouldn't do this or even allow our server to accept such property changes, since editing and entering just any author-name is not the way to go.