You can add as many breaker sizes as needed to Design Master Electrical.
To add a breaker, use the Wire Sizing command.
Ribbon: DME: Customization->Project Customization-> Wire Sizing
Pulldown Menu: DM Electrical->Customization->Wire Sizing->Edit Project List
The grid at the bottom of the dialog box is your breaker list. Add the new breaker using the New Breaker button.