Certainly you can, as long as the registrar you choose gives you modification access for your domain.
If you purchase a new domain of your own somewhere other than through Mumbai Hosting simply make sure you pick a reputable domain registrar who actually makes you the owner (Registrant) of the domain you buy, and that you will have access to modify the DNS for your domain.
After you get your new domain registered (purchased), set the DNS to our nameserver info as specified in the new account email we sent you.