Werner - thanks for your question. However, it seems like a question specific to Xpedition, so I am going to move it to the Xpedition>Layout community. This community is more about general community questions or suggestions.
I don't see this myself. In VX.1 we actually added naively the ability to select a number and while in plow change to this layer adding vias if needed.
So in the end I do not see this issue as you describe with plow being able to change active layers by simply selecting a number key.
The only time I have seen this not work, is when the Num Lock was not enabled for the system. Please verify that in Notepad when you select a number on the number pad, it actually shows a number.