code to activate pro sites?

Hi,

We're using a 3rd party payment sytem and a 3rd party membership plugin but want to include pro site functionality as part of our membership.

Is there anyway to give our members a special code (maybe a coupon code?) to use on the pro site upgrade that would give them access?

Or maybe you guys know someone who we could hire to develop this feature? We just want to avoid doing a manual upgrade for each new member...

Appreciate any suggestions...