You can definitely keep your preexisting domain and use it for your OPP site, we do this all the time.

When you activate/pay for your site, just type your domain in as the URL you want and then indicate that you already own it. When we go to set up your site, we'll send you an email explaining how you can point your old domain to your new OPP site.

There's no need to transfer the domain to us or elsewhere --- you can maintain ownership of it at your current registrar. You'll simply be making a small change to the domain's settings, called "changing the nameservers."

If you are currently paying for a hosting package, feel free to cancel that, as hosting is included in the cost of your OPP website. Make sure, however, not to cancel your domain name...