Pro Site Level and Gravity Forms Registration

Howdy all,
I'm looking at using Gravity forms for the registration process as I would like to collect a bit more information from the client and display some other stuff as well. Is there is a hook I can use with a custom field so a pro site level can be selected during registration.

I was thinking letting the user select the package they would like first and then go through the registration process.

I've seen a few posts discussing gravity forms and membership but I'm thinking it works a little differently to pro sites.

I'm may also have a question about the affiliate links but will wait and see how the referal url option works

Cheers,
D