domain mapping and cpanel

This is my setup:

I have a multisite installation lets call it http://siteA.com

I have a blog on that site http://siteA.com/siteB_blog

I have a cPanel installation of siteB.com so that I can have email accounts for siteB.com

How do I map http://siteA.com/siteB_blog to http://siteb.com

The domain mapping feature works fine, but it points to the cPanel hosted site of siteB.com

What changes in the cPanel of siteB.com do I have to make to ensure only the web traffic goes to the multisite installation.

Both of these are on the same IP address.

Thanks,

Anthony