If you purchase a domain, you can then point it to your leet domain.
In the DNS Settings, you would have to:
point the CNAME to YourServer.pocket.pe
point the "A Record" to the numeric IP of your server.
But please note that you will not be able to advertise your new IP on the LEET system, as moderators do not have time to verify if the domain matches LEET Servers.