Yes, you can. There are two options, 1) to transfer the existing domain to us, or 2) leave your domain with your existing domain name provider and point your domain name to our nameservers.
With 1) we would invoice you for the hosting and also the renewal of your existing domain when it is due.
With 2) The nameservers you need are in your account Welcome email, and we would invoice you for the hosting only. You would be responsible for renewing your domain with your existing provider when it becomes due.