You can just rivet a track over the slot, or over the groove. You'd then have to install lazyjacks, get a new sail or severely modify yours with slugs or cars and have reef points installed. Then you need to modify the boom for slab or single-line reefing, change the line routing, install a whole bunch of extra turning blocks and clutches and deal with all the problems with that system.
Probably much simpler, and certainly cheaper to deal with the problems of the system you have and continue using it. If furling is hard, you need to service the in-mast furling gear (the winch and gears at the bottom as well as the top swivel), check the rest of the system for stuck turning blocks, worn/baggy sail, broken outhaul car (ours had a
broken wheel) and make sure you're using it correctly.
We have often furled or reefed our in-mast main in high winds and it's been working fine, although you need to pay attention to the boom angles, let the sail come in from the port side and not have it powered up. When I got the boat, the in-mast furling had never been serviced and was very hard to use, after servicing the sail is easily furled by hand in most conditions, just like the Genoa.