How to add profile fields on Ubercart 3 checkout page?

I used to use the uc_profile module to add user profile fields to the D6/Ubercart 2 checkout page, but I cannot find a suitable solution for that same functionality in D7/UC3. Can anyone guide me toward an alternative solution? I need to add things like "birthday" and "shoe size" to the checkout page, and I need those items to be displayed on the users' profiles.

Thanks!