They wouldn't need the PCB designer. They would need to suborn (employees of) the PCB manufacturer and the assembly house. These will be supplied with the PCB layout (in Gerber or ODB format, for making the PCB) and the bill of materials and pick and place file (for assembling it). With these files you can reverse engineer the schematic and carry out the modifications described. The PCB manufacturer might well be a sub-contractor to the assembly house.
Most electronic engineers don't have the software tools for or experience of sophisticated reverse engineering but there aren't any major conceptual barriers. They would have to go from the geometry of the tracks and pads to a connectivity graph (very automateable), then collect the pads into footprints for components (probably partly automated), then identify those components and the functions of their pins (easy with a complete BOM) and then workout the circuit function (should be straightforward for standard parts and circuits).
There might not be automated tools for making the desired changes, in which case they will have to manually draw the new track geometry on the Gerbers, add the parts to the BOM and pick and place files and change or nobble the test criteria / files. Hard work but quite straightforward.
These skills will be developed by people doing legitimate industrial reverse engineering as well as espionage / intelligence. I would think there are also unfortunate cases where firms have to reverse engineer their own products after losing the original files.