As someone who's recently translated a bunch of FDS games, I'll bite on this one.
The problem with converting them is that the format is quite different. Regular ROM games are simple: if they don't use mappers, they just have the entire 24/40kb available for the NES to access, and if they do use them, the MMC switches parts of the ROM in and out using certain instructions.
FDS disks are just that: disks, with files like you'd get on a PC. Instead of one big ROM lump, you have different files. Plus there are two sides to store them on, and when the FDS was released it was quite a lot of storage (before MMUs made it redundant) and of course the innate ability to save.
The way the FDS works, it has a BIOS for loading from disks, unlike the NES which doesn't need one. Some games, like Mahjong, whose translation of mine I converted from ROM to disk, will just load the entire game into RAM at the beginning. Others, like Knight Lore, use both sides and have data for each level in different files, which are all transferred into RAM at the same point, overwriting each other. The files can be any size, and can be written to wherever you like in RAM.
This is very different from a ROM using bank switching. MMCs switch banks in certain specific amounts, and it happens instantly. Whereas a file could only be under a kilobyte, a bank has to be at least 4kb, usually more, and is a set size, so often you'll get wasted space.
Not to mention in all this that instructions in the games will point to different locations if you changed to a ROM. It could take a lot of reprogramming to make a larger FDS game work in ROM format, and require one of the more advanced mappers.
At which point you realise that every emulator can play them (even Everdrives can I think) so you wonder what's the point.
TL;DR - it's not as easy as you hope.