2
This can be done, but keep in mind that this will require hacking the XOOPS core which will;
1) Complicate your XOOPS upgrade path
2) Limit the support people will be able to give you because you have a customised installation.
Also I am pretty sure (don't quote me on it) that XOOPS 2.2 will have custom fields for the user profile. If you are ok with these ponts, then have a look at
this and
this.