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I believe XOOPS supports authentication against an LDAP database. If Coppermine does too, then you could go that route and have one LDAP database that both systems used for authentication.
There is a port of Coppermine to XOOPS called xcGallery. That would let you essentially put Coppermine as a module inside XOOPS, though I don't know if an "upgrade" is possible, ie: you might not be able to move your existing Coppermine content into the XOOPS module.
Good luck,
--Julian