Yes, you can purchase a Web hosting with us even if your Domain name is registered with any other company. having a Domain name with us optional. You can buy web hosting plan from below links.
once you have purchased required Web Hosting account, you will receive a Welcome Email containing Name Server Information of our server, you will need to change/update name servers in your domain name.
Note: if you are using any other service in your domain name e.g. Remote Email service or subdomain or SSL certificate, please contact us before you update your Domain's Name servers, otherwise all other services will stop working.