You could, and I am just going off the top of my head here without studying the bikes schematics, I would say you would need two relays one for each light ( Left and right ) since you do not want to interfere with normal signal function. Cut the wire to the left signal light. Connect the wire coming from the the light to terminal 30 and the other wire to terminal 87a. . (which is NC) Now do the same thing with the second relay and the right signal light. At this point the signal lights should work as normal again.
Next, tap into the red wire from the back off module and run to terminals 87 of both relays.
Connect terminals 85 of both relays to ground
Connect terminals 86 of both relays to either the yellow wire of the back off module or the same terminal on the optional switch the yellow wire is connected to.
this should keep the front signals operating as normal when the optional switch is off but switch the lights over to the back off module when the optional switch is on.