Adrian's suggestion might work but would also give access to all domains on the system.
I believe "nested domains" is a feature the developers have built but is either still in beta or is a higher membership level feature.
The work around we implemented for the single person that needed access to multiple domains was to just give that person multiple user accounts, one for each domain he needed. If unique usernames isn't enabled, it can be the same name under each domain needed, but would require enabling the domain selection on login page, or the "admin" choosing the correct domain name to browse to.
Our system has unique users set to global and typically build the username as the person's email address. The one multi domain admin we have at the moment is lucky to have an easy name to manipulate and multiple email aliases.
customer1.domain.com has user
mike@hisdomain.com
customer2.domain.com has user
mike.lastname@hisdomain.com
customer3.domain.com has user
Mlastname@hisdomain.com
Neither solution is ideal, but for limited use cases, it works ok for us.
Another alternative could be to use a username such as customer1admin, customer2admin under each domain if email addresses are cumbersome.