It’s not easily doable at this point no. I’m sure it could be done with some symlinks and possibly redirects but I don’t know the best way to facilitate this off the top of my head.
Add the www.aaa.com (customers site) as a pointer domain into the resellers account (bbb.com). Add the above to the .htaccess file in the root of www.bbb.com and aaa.com will load files from /home/bbbcom/public_html/aaa.com/
You may need to alter slightly the rewrite code, or make other small adjustments. The emails should work okay, but obviously stats etc wont as you are not setting up a separate hosting account for aaa.com.
You obviously would need to add the last three lines for every domain that was added to the resellers account