From what I can see anyone who moves to Georgia up to the runoff can vote in it : https://faq.georgiavoter.guide/en/article/requirements-to-vote-in-georgia
I have confirmed that there is no restriction on voting in the General Election for the runoff, they ARE accepting new registrations : https://www.wtvm.com/2020/11/09/key-dates-senate-runoff-georgia.
DEFCON 1.