Yes, Supermetrics does support getting the custom metrics and dimensions from Google Analytics!
They will be presented in the field list as "Custom metric ##" or "Custom dimension ##", instead of the specific name you gave them
To find out which number was assigned to the field, go to the Google Analytics UI and open the Admin panel. Under Custom Definitions, select Custom Dimensions or Custom Metrics. Check the table for the field you wish to use and then check the "Index" number assigned to it. This should be the ## value in the field name we use.
For example, the image below shows some custom dimensions set up for a propery. If one wanted to use "Test1" in Supermetrics, it's index is 1, so it should be the "Custom dimension 1" dimension.