4
akitson is right, Registration Codes certainly provides this functionality in 2.0.x of XOOPS. We are happy to answer support requests in the forums in the Formulize/Reg Codes area of dev.xoops.org (linked above in akitson's message and below in my sig).
My best advice...read the readme for Registration Codes carefully! There are several steps to setting up the custom form.
Good luck,
--Julian