Are you trying to add draft to the holes or to the box?
If it is just a box as you describe, select the top surface (so that it highlights in pink), hold down SHIFT and select the edge of the top surface that it shares with one of the surfaces to be drafted. This is how to pick "Loop Surfaces", which are the surfaces that are adjacent to the first surface that you chose, bounded by the selected edge.
You should be able to draft this without a problem.
Where are the holes located on the part? If any holes or pockets are into the side surfaces of the model, then that is a reason why you don't want to add draft to the initial extrusion. Unless you went and created a number of datum planes (from the sounds of it you don't like creating extra features), you would not be able to place these holes or pockets on the side surfaces, since the normal direction of the surfaces is no longer perpendicular to that of the top and bottom surfaces.
A draft feature is a manufacturing detail. You should design the part in its "theoretically perfect" state before adding draft. This will allow you to apply true design intent to your model. You then add draft to facilitate manufacturing.
Solidworks' draft with extrude functionality is also limited. Your neutral plane is the sketching plane. I admit that it saves a few seconds and a feature, but once you know how to use Pro/E's draft tools, there is nothing that you can do with Solidworks that you can't do with Pro/E.