Yes, you can purchase only hosting service with us. It is not necessary to register or transfer your domain to us to use our hosting service.
If you keep your domain registration with other provider, you just need to change the nameservers of your domain to point it to our server. You will get the nameservers information (NS information) from our login details email.
In case if you do not want to change the nameservers and you just want to update A and MX records, you can submit a ticket from your client area to obtain necessary DNS records.
