Domain mapping on primary site

Hi, ok this a rework of another question so it is clearer what the problem is.

I have domain mapping setup and working except on the primary installs site.

I installed WP on mycompany.com converted to WPMU and created new sub sites.

For the mycompany.com i did not need to setup any domain mapping, for the sub sites i did and it works great.

Now i want to have another domain point to mycompany.com, but in the mycompany.com wpmu control panel there is no Domain Mapping option?

How can map this new domain to mycompany.com?

All relevant dns settings to point new domain to mycompany.com are correct, but when i try it just loads the wp-signup.php page