You may not need an iterator - it all depends on how many fields you are adding. If you are just adding a handful of fields to one file, I would suggest the pragmatic approach of just having several Add Fields functions chained together in a row in Modelbuilder (one after another).
Add Field - ArcGIS Help (10.2, 10.2.1, and 10.2.2)
How many fields do you need to add?
Chris Donohue, GISP