So you don’t want the dropdown menu as a popup when running Dynamo, you just want the dropdown menu in the properties window of the family type in Revit it sounds like.
For text, a dropdown menu is autogenerated when that text value exists for that parameter somewhere else in the model. So let’s say I have a comments parameter on FamilyInstance1 set to “1”. If I go to FamilyInstance2’s comments parameter, there should be a dropdown menu that contains “1”. If I want “2” to show up as well, I need another FamilyInstance that has it set to “2”.
The same method works for family types.
There is no need to do this through Dynamo. The only way Dynamo might be useful is to autogenerate two new ‘sacrificial’ family types of the same family with the parameters already set to “1” and “2” so that every new family type will have that dropdown menu.
Edit: I think what you are actually trying to ask for is Lookup Tables when you mentioned Excel file. That isn’t a Dynamo thing, that’s a Revit thing.