Looking through the registration.php page and then looking for where the defaults get set, I see that class_dm_user.php gets used to set the registration defaults. Perhaps look into calling this page (in the includes directory) and using the function set_registration_defaults will help you out. That function, however, is included in "class vB_DataManager_User extends vB_DataManager". So, you might want to look into using the datamanager to include this.
edit: This article may help you out:
Add new Users (automatically)