I once made such a system and actually got to playtest it, but concluded that my creative skills are better suited for the videogame format. Unfortunately, I don't have any of the documentation on the computer I am currently using and the paper documents are god-knows-where, but in general found that the adaptation is difficult to say the least.
From my experiences, I felt like the major obstacle is not necessarily in defining the game-play or combat-related rules, but rather figuring out how to use them. Will you just plug the combat system into a different system? Will you be following a chapter-like structure or in other ways restrict the players to following a set storyline in a linear fashion? How much balance is there between the roleplaying aspects and the combat? And perhaps most importantly, what advantage in depth or interest does the use of FE systems provide?
I feel like these things are something that should be defined first, and then the flow of combat should be designed in such a way that complements your base idea. If you are planning on borrowing just a few ideas from FE, they should be fairly easy to incorporate into any larger system. However, if your goal is to some degree emulate the FE-style combat in more detail, have the weaponry and character stats the games are well known for, plan a combat-heavy campaign, etc., you will need to seriously consider how to make the features more interesting in the tabletop game than they are in the video game format. My players generally found that the system is interesting, but having to deal with the rules of FE manually was much more tedious in tabletop format and did not really provide additional strategic depth over alternative combat systems.
Perhaps you shall succeed better than I did. Hopefully my mistakes were of some use - good luck!