I have just successfully done tis using DS 7.5 HF5. Here are my steps:
1. Pushed SMA to Windows 7 Enterprise workgroup machine.
2. Deployed the DS Plugin to this machine.
3. Created a system configuration (Settings > Deployment > System Configurations).
The computer name was left alone, the Domain radial button was selected and the domain FQDN entered, the domain credentails were entered, and the Network Adapters tab was left alone.
4. Created a new Apply System Configuration task, and selected the system configuration that I had just created from the dropdown list.
5. I targeted the workgroup machine using the tasks Quick Apply function.
6. Task ran successfully and rebooted the machine.
7. I was presented with the Ctrl+Alt+Del screen indicating that the domain join had worked (workgroup machines usually display a user selection).
8. I then used the Switch User function to enter a domain users credentials.