Entering a UPS or FEDEX COLLECT Account in shipping options needed.

I would like to be able to enter a UPS or FEDEX COLLECT Account # when I pick a shipping option by adding a field on the shipping page. For now the account is entered in the Comments field. Is there a way to do this . Sometime the customer forgets to enter the account no. very frustrating.

anyone want to reply with the file to modify or code etc??