I need to modify the registration module (I see the ascx page in the controls folder - User.ascx) and add a field.  It is an ID field called ARID to link to another registration in another database.  When the user registers, he can enter the ARID number and then I have a link back to info in the other database.  Or better yet, I could have a module I build that the user would first enter the ARID number and then go to the registration page and I prefill the info on the text boxes for the user if they are in the other database. 

Either way, I need to modify the registration page. has anyone done this and what are the steps I need to go through.  Is this part of the dotnetnuke project or a module?