There is no way to add nodes.
This brings up something though, that hopefully the software will do one day: If you make cuts on something in the part editor, thus changing its shape or creating points on it, the nodes should be regenerated for that piece based on the new shape. As it is now, if you take a square and cut it into a triangular shape, you cannot directly measure it because it still has the nodes of the original square, floating off in space. The only way to measure it is to take it to the line drawing editor, where it will have nodes in the proper places.
Can we look forward to nodes being regenerated for PE-cut parts in the future?