In part it depends on what you mean by editting.
PTCs parametric modeler is a procedural modeler that also allows changing named values. Procedural means that there is an order to the operations needed to build the model.
Step and IGES are boundary models. There are no operations in the models, and so there are no procedures.
In particular, Step and IGES don't store the 2D geometry that was used to create the 3D model. There is nothing to 'edit' in the Pro/E parametric sense.
The reason you can use them, even if changing them is difficult, is because most of the time you would not change them. Standard hardware, an engine, and electrical components; are all items that are useful to design, but don't need much change. That said, you can use imported models and add and subtract material.
You can probably borrow the hole by copying the surface geometry and pasting it where needed.
