Active Directory - Additional Domains, Customer Naming Conventions

We are a service based company. We have a few custom web interfaces we have built for our customers to login. They login to view open tickets...etc.

All is integrated with Active Directory. So when we add a new customer, we have a naming convention of firstfourofcompanyname.firstname.lastname. We would like to add their email address, like bob.lastname@domainname.com, but when I add a new user in Active Directory it only works for our domain name (domain.local).

So my question is, can I add all of our customers domain names?