Brian I cover this in depth in my guide so I will not repeat it all here but the basics are yes you can add listing fields at any time but just ask this one question first as it will give you the correct answer most of the time (there are exceptions) > "Ask if this Sub Category belongs to one or more Parent Categories to determine if you should create a Sub Category or a Listing Field."


In short if a sub category belongs to multiple parent categories it is most likely a listing field, if it applies to one only it should be a category.


(Just my views / take it or leave it)