I have a Leclerc floor loom, and have often thought of how to increase the number of harnesses on the loom (it has 4 harnesses) by using an auxiliary frame. This would be free standing, not actually connected to the loom except by the warp, and could have 2 or 4 additional harnesses, either jack or counterbalance. Two upside down T shaped uprights, with braces to keep everything square, placed outside the footprint of the loom, with the new harnesses to the back of the main harnesses, unless it is a counterbalance system, and then half would go behind and half in front of the loom harnesses. I'm hoping to avoid buying parts from Leclerc (so expensive) and instead buy parts from the hardware store. Though if my green woodworking skills were up to snuff then I could probably harvest the wood from my property. I already have lots of extra heddles (the eyes the individual threads of warp go through on the harnesses). There would have to be a board going across the top connecting the two uprights and to which to connect the lifting system for the new harnesses. I'll try to do a sketch of my idea.
Sounds like a great loom. Have you any photos to share?
Increasing the number of shafts will decrease the shed size a bit, but it is possible. I suggest attaching them to the frame somehow instead of having them float, that way you can keep them close to the other shafts and don't lose as much shed size. Once you have a photo, I can see better which style of loom it is and make suggestions which frames would work best with it. I bet a lot of it can be made from salvaged materials. You've got the heddles already, so you've finished 90% of the work right there. You're well on your way.
Personally, 4 shafts are the perfect amount for me. There are so many complex and beautiful patterns that can be done with just four that I can't imagine going back to eight or 16. Also, it hurts my body to thread anything more than 4 as I have to reach and see further to thread the heddles. But that's me and my style. Each weaver is unique and it's important to have the loom that's right for you.