Yes, you can. See QMC->System->Set-up->Directory Service Connector->Select yours->Custom Directory. Add a DSC definition, select apply and a Users tab will appear. Select the Users tab. It's divided into two regions: Users and Groups. Define your Users and Groups in the corresponding areas.
Two things to think about:
You'll need an Enterprise Edition server for this to do anything more than just look back at you (DMS required)
You cannot mix definitions from different DSC's, like for instance AD/LDAP Users in Custom groups.won't be possible...