You might want to change the domain registration button link url of plesk to your own.
Either simply to switch it to the registration service you’re prefering or to make some money from your customers domain registrations.

This is to change the url of the “Register Domain Names” buttton:
/usr/local/psa/bin/panel_gui -p -domain_registration_url

And this one is to change the url of the “Manage Domain Names” button:
/usr/local/psa/bin/panel_gui -p -domain_management_url

http://download1.parallels.com/Plesk/PP11/11.0/Doc/en-US/online/plesk-linux-advanced-administration-guide/index.htm?fileName=68568.htm