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Many, Many moons ago there was a convert script, I think it converted to XOOPS 1.2/3 or something, back in the early days of xoops.
Not sure where this would be now though, I once had it as I converted, but not seen it in a long time.
I don't recall anything newer than this, and it would take a lot of work upgrading through to the latest xoops.
Maybe worth paying someone to do a conversion script for you, if the database means a lot.