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There is not a straight conversion.
I know you don't want to hear this, but the best way would be to make a test install of XOOPS and print out the database tables. Compare them to your existing website database. Make adjustments as needed and insert the old data into the new database structure. It's tedious. It will take some time. Not every single field will match and you may lose some data because the two programs are different.
I moved my website over from phpNuke a few years ago. At that time, I had no knowledge of mySQL. Reconstructing a database from scratch was quite frankly a scary proposition. I learned a lot. I made some mistakes. It took me more than one attempt. In the end, the only real hard part was the forums. I can't say that I really know mySQL now, but I am comfortable enough now that I will make changes to it and know that those changes will do without fear.
Good luck.
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