I believe chuck may have written a script to map exisiting PP users into your forums user table, (scripts/whatever all depending on which forum you use) but i'm not to sure about this.
this is what i would do..
I would install your forum anyways ask your users to register and then install photopost (integrated with forum of course) then i'd take the photos from existing users/photopost and upload them (set as "username") to the gallery...
Long process maybe
- but maybe chuck has a faster solution, good luck