I've also had a Shark Pro for several years now, and have looked into adding limit switches more than once. It would mean abandoning the Shark controller, replacing it with either a TinyG or something bigger/better that supports limit switch inputs. Then adding a minimum of 5 switches (X+/-, Y+/-, and Z+) and 2-lead cables running back to the controller from each. Not impossible, but not a simple add-on kit either. All limit switches can be mounted on the gantry with the Z+ being farthest up the cable chain.
I understand why you want limit switches, but also haven't really needed them on either my Shark or the one I oversee at work. Their latest controller software gives a plan view of the toolpaths you have loaded into it, and dynamically shows where the bit is as it jogs/moves around. Once I've set ZYZ=0 I simply jog the bit to the limits of the preview screen to verify I won't be exceeding any hardware limit, and if I do then I'll re-clamp the work closer to the middle until I'm sure no limit will be hit while cutting.
I also have a Probotix Meteor with limits and e-stop. It is run by LinuxCNC on a Linux PC. You could buy the Linux PC and controller from Probotix to run your Shark, but it would require changing all the cable plug ends to match jacks on their controller box.