Custom Field In User Registration Portal

Hello,

Is there a way I can add a custom field during user password registration in FIM 2010 R2? I want to have a field to set a User PIN where user can enter a value and it can be used in future by user or helpdesk when user is not able to answer other secret questions.  I can not add it as one of the questions in QA gate as I don't want it to appear in initial Q&A when a user tries to reset his password. It should only appear when he is not able to answer other questions.

Any ideas are welcome

Thanks,

Nagendra