I had that idea years back. In my opinion you have three options:
1. Buy a new rifle made to take hi-cap mags.
2. Modify existing hi-caps for the 100 or modify mags for the 100 to hi-cap.
3. Modify the 100 to take hi-cap mags.
I'm not familiar with the Win 100, but I do know that the Rem 7400 shouldn't be used for the rapid fire generally performed with hi-cap semi-autos. The SKS, while worlds better, isn't as good for sustained rapid fire as the AK reliability-wise.
So there's the case for a new rifle.
My impression is that Win 100 mags are scarce and expensive. I'd recommend against moding one of those mags to hi-cap. I'd suggest instead considering hi-cap mags for another 308 rifle, ie the FAL, G3, or M14. You could get a few of those for the price of one Win 100 mag.
Moding the Win 100 would probably be even riskier. I'd be hesitant and I almost have an engineering degree. If I did mod it, I'd prefer to make a new part that would replace an original and not actually modify the original Win 100 parts. I have developed a hi-cap mag mod for the Saiga 308, but that's a current production gun on a really simple design.
My honest suggestion would be to keep a classic rifle as it is. If you want to experiment with hi-caps for it, leave everything original intact and just modify surplus mags. The graet thing about the ban being dead is that you can modify mags to your hearts content legally, and not worry about screwing up a grandfathered pre-ban one.
Hope your Win 100 treats you well. I'd absolutely love to have one of the few in 358 Winchester.