Jo Ann,
I think what will work best as most domain registrars let you park your domain there and you can just redirect it to the subdomain without pointing servers as you aren't actually moving it to another host just redirecting traffic.
Yeah I was thinking that just a few minutes ago lol *hums the Twilight Zone Theme*. That's how I used to use my own domain for many years with all the bazillions of free sites I've used instead of parking it anywhere I left it at the place I purchased it from which is usually Mydomain.com and then used their free service to redirect the domain to the site. That may be an option also which might be simpler for sure. I'll still need to know how to do it the other way though for those that want to park it with us so I'll still look into all that, but the redirect may work much simpler in the end.