I'm not quite sure what you mean by a "whole farm test plot".  Do you mean you want your entire farm to be considered one gigantic field? 

The Ag Leader display will be expecting that you split up your operation into Grower(s), whom each have farm(s) within each there are field(s). You can just have a single Grower and a single Farm with each of the fields under that farm.  If you had a small operation you could have just one field.

This may be what you have in mind.  If this one "field" was large there could be other complications.

I farm with my nephew and a neighbor.  I have just one Grower and one Farm with about 17-18 fields under that.  Our operation could be organized several different ways but this way suits our purposes.