@hamadesign – Ivan Atanasov here, part of the Senior Support team at SiteGround.
What you are describing is possible. You can register your domains with the registrars of your choice, and still point them to the hosting company you prefer (in this case – SiteGround).
You need to add the domain in your hosting account. Then contact the registrar and request assistance for changing the NameServers so that the domain can be pointed to the server housing your website.
If you contact us from your SiteGround User Area -> Support via a ticket, it will be our pleasure to guide you along the way.