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The Atlantis Trilogy started out this way first as the Complete "X" series and then as Arcanum etc.If you got in your time machine and went back to 1978-1980 you'd find a couple of games that we now think of as stand alone systems but really started as 'patches' for bolting onto D+D. And, of course, D+D was much more loosey goosey back then, so it was easy to find places where you'd bolt them on. A creative person could do the same now. That said, I think the more natural approach is to use TFT as a coherent complete system and just adapt setting materials, new spells, etc. from other games, rather than trying to merge combat and magic from TFT with stats and skills and so forth from other systems. Yes, it can be done as a sort of white-room exercise, but is the effort really worth it when the combat system is 2/3 of the whole game and the other 1/3 is sitting in front of you in the same book?
I believe Rolemaster started that way as Arms Law and Claw law, alternate combat systems for FRPGs(D&D)
Can't think of others but I'm sure they are there.