Hi Beenice, welcome to X-Cart! Please start by reading the forum rules found here:
http://forum.x-cart.com/viewtopic.php?t=6489 and post your X-Cart version in your signature. Edit profile, enter version, enter password, save. Thanks!
Now on to your question....it is certainly a possibility, but I don't know of a mod that has done that. So are you saying you want the customer to be directed to either a registration form or a login screen as soon as they hit the 'buy now' button?