Getting Old WPMU Account to Display in New Install of WHMCS

I've got a site on my WPMU that was built before I had WHMCS configured. Is it possible to create the client record in WHMCS and get their WPMU site listed as a product in their client record in WHMCS? I want WHMCS to be able to take over the billing for the site. I didn't try creating the product in WHMCS because I didn't know what would happen.

Thanks for any insight you can give me on this one.