Multi domain but only one admin area

Hi,
I have just made Multi Domains working.

My problem is that the domain foo.tld have the admin area on foo.mymainsite.tld

Is possible to have the admin area on mymainsite.tld/wp-admin ?

I don't like my customer view foo.mymainsite.tld.
And mymainsite.tld is the only https.

Thanks you.