Having a plugin modify xwiki core structure.

In building my google docs plugin, I need to modify the structure of the user signup / details page (adding google username and pass). I would like to know if this can be done without my modifying the core structure of xwiki. To clarify: can I make it so that my plugin, when put into use, adds the two fields to the signup / details page? This would solve another small issue that would intervene if modifying the core - when you don't use the docs plugin, you get 2 useless fields in said pages. Any ideas on if and how I could do this? Thanks, Radu