Using another domain to access a multi-site subdomain, can't figure it out

I need help on using a new domain to access a drupal site I have created using the multi-site functionality.

For example:

My primary domain is example.com and I run a primary site, as well as a site mr.example.com and joe.example.com. The second and third sites site in the folders sites/mr.example.com and sites/joe.example.com.

Joe then buys his own domain, example2.com and he would like to use that domain for his website.

My web host (hostmonster) allows me to park a domain to direct to my primary, but I don't know how to associated example2.com with joe.example.com. It also allows me to point example2.com to a folder in my public_html directory, which is where my drupal sits. It doesn't let me go to deeper folders than that, which is where /sites/joe.example.com is, and I don't know if that would work anyway.

I can use a redirect to go from example2.com to joe.example.com, but joe wants the user to not see joe.example.com and just see example2.com instead, so that doesn't work.

I'm new to Drupal (and loving it, BTW), and I searched and looked at the various system files, but didn't see anything that spoke to this. I looked at the Domain Access module, but it says it is for multiple sites that use the same database, and I use separate databases for all three sites.

Any help is much appreciated.

Don