Upon a successful join, CCBill can call a script and send whatever data you ask for (minus card numbers, etc) to a postback script anywhere you want. You should be able to write a little postback script that will take the join data and add it to your X-cart database.

Alternatively, you can collect the data (again, minus the credit card info) before sending the surfer to the CCBill join form, and use a little script to write it to the x-cart database, then pass the data to the CCBill join form (so they don't have to re-enter it) where the surfer will enter their card number and expiration info, etc.

If you do it the 1st way, you'll get only members who actually succeeded in joining. If you do it the 2nd way, you'll also get the declines. Depending on what you're doing, each method has its advantages.