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You might be able to backup the xoops_bb_forums table as an .sql file, reorder the lines in that file, and then restore the table from the .sql file. I haven't looked into that.
Or if you're into hacking, you could add a sort-column to the table and change the MySQL query that retrieves the forums to use that column.
After you've upgraded to newBB 2 (which is not recommended yet for production sites), you'll be able to change the order whenever you like from the newBB admin page.