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If you mean convert the entire application phpBB to a XOOPS module, then...
It depends on the complexity, and this would be big. If the mod has it's own user authentication system. If it uses a database that can be easily merged. the coding part is also big. You can use the builtin routines of XOOPS for things like forms, database access, etc - or you can use what's in phpBB.
there are just a lot of variables, so it's not as easy as yes or no.