Interesting; well, I can't say I've done that, but off the top of my head I would say it "could" be done but would require a little work. You'd have to migrate the user data from vb
into the pp_users table and make sure you have a sync'd set of usergroups before you do this.
When moving users, make sure the userid stays the same between the two systems. Then change the vbversion variable in the database to "photopost" (from "vb3") to use the photopost integration.
I *think* that would do it - just move over the users data and then change the integration type.