We want to use another domain name of ours as primary one for our account. Do we need to transfer that domain from the current registrar to Siteground?

Transferring your domain name to our registrar is not a must thing to do, in order to use it with your SiteGround hosting account. You can only point the domain name to the name servers of the server that hosts your account.

A good reason to transfer your domain to us is to manage it more conveniently together with your hosting account. With SiteGround, you will have full control over the domain name from the Customer Area.

You would be able to change the administrative contact information such as Organization, First and Last Name, Email and Living address, etc. Also you could edit the DNS settings, change the domain name status and check your domain EPP code.