Pole barns are tricky to retrofit with stuff on the walls since they don't typically have any structure in the wall except for the occasional nailer. 
You can do the grafting or if you want a window, build up a bit of support underneath like Nathan shows. Since there is no roof load to support you can skip headers and just secure to that top board with some rafter type plates and then frame the opening with some 2x's. It is nice to double them up around any opening to get the extra beef for installing any trim.