[Exim] exim3/spamassassin with userconfigs

Hi,

I need your help with the following topic. Thank you in advance for your
time :)

On an exim 3.35 system (debian) I want to call the spamassassin client
(2.55) spamc with the option -u $mbox-name.
I want to use that for user-configurable options of the spamassassion.

So my problem is, how can I invoke a "filter=spamc" in a state of the
mail-delivery process where I can tell the "filter" the mbox-name the mail
should then get locally delivered?

Is it really necessary to use procmail or is there some faster trick
possible for this matter?

Does someone use spamassassin with userconfigs of users, who do not have a
home-directory on an exim-system?

Thanks,

Dennis Breithaupt