Adding hosts to a host group in Satellite 6.5

I normally provision new hosts via Satellite and have set up several different host groups to satisfy most of the required information for creating the host. I also use the host group to run different jobs / roles. I've recently added some existing hosts not provisioned by Satellite and would like to include them in some host groups but I don't see a way to do that (not through the UI at least). Is there another way to do that?