I am not fully certain I understand the dilemma. On the surface, one idea I had was to recommend that you list your franchise subset in a Google Sheet and add it in as a separate data source to the dashboard. Without seeing a sample of your setup this is very difficult.
Could you possibly create a dummy data source that has just 5-10 rows so we can see your dashboard layout?
Hi James - To make it simpler, I have 2 dimensions, Site and Franchise. I need to add the Franchises into the Site list so they all sites and franchises are listed together in the same dimension.
Unfortunately it sounds like you want to essentially create data that doesn't exist. It seems like you may need to Union your data and added a flag so you know which is which (Site v Franchise).
SELECT 'Site' as Source,
Site as DimensionFilter,
SELECT 'Franchise' as Source,
Franchise as DimensionFilter,
Of course you'd then have to take into account potential double counting.