272391
Easy: XOOPS tables all have a prefix, so you can have multiple (xoops)installs in the same database, each functioning as it's own site. So, in this theory, you can install YABB, as long as it doesn't have the same table names as your XOOPS install (which would be strange, as all XOOPS tables have the prefix you said they should have). The database can hold a large number of tables, so that shouldn't be a problem.
Herko