How to prevent PositionHandle to become FreeMoveHandle when shift key is pressed

I use a PositionHandle in my custom scene editor, and I would like to use the shift key to allow user to do a specific action when the handle is dragged.

Problem : shift key is already used by PositionHandle, that become a FreeMoveHandle while it's pressed. Of course I don't want that.

I have already tried to Use() the event when L-shift or R-shift is pressed but it didn't work. Any idea to deactiviate that behavior?