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The way I would do it is to compare the existing database structure and semantics to the XOOPS database, and figure out how they correspond. This might require studying the scripts that manage the databases and any available documentation. Once that's known, then a custom script, or a manual process if the database is very small, would be used to port the data.
It could be an easy or a very difficult task, depending on how similar the databases are, how much data you're willing to discard, etc.