Light fixture powering other lights / garage door opener?
From your description the wiring is normal not "jerry rigged". You can add a new wire to the switch but from the wire count you may not be able to fit everything back in the box and your wire fill would exceed code. It may be better to purchase a 2 gang old work box cut the hole the right size for the box pull the old box and install the new 2 gang box. It may be easier to add a 4x4 box in the attic to make your connections to the outside lights and new feed from the 2nd switch. All connections need to be in a box. The 3 sets of wires in the original switch box are possibly the hot feed, the switched feed to the lights and a hot feed to outlets on the wall this would have been a normal setup a few years back. Remember to turn the power off prior to doing the wiring changes and good luck.