Pay to blog + Credits?

Hi,

I wonder if it is possible to somehow join or merge the use of pay to blog and credits on wpmu? I would like to provide something like, if anyone signs up for an account by paying a certain amount, e.g. $20. He would get 10 free credits accompanying with the account.

Is this possible with any of the plugins, or do I have do customization to make that work for me?

Thanks.