Adding a Layer to an almost ready PCB

Hi,

I have to rework an old double layer PCB project adding some new features. The problem is that there is no free space to add some new tracks. How can I change the PCB to an 4 layer board to have additional space?

Is there a feature to add an 0Ohm bridge to cross signals without changing the net name to and from this resistor and without having a representation of it in schematic?