Yes, I can always add them via POI's, but the Nav Pro (not sure about the other Nav), has a feature whereby when the cars running low on fuel, it will ask if you want it to find the nearest fuel station to where you are, and then searches on the Brands you have selected.
As I said, all seem to be there, except BP. So even if I add as a POI, the Nav won't use this as part of its search criteria, as these are inbuilt.