About 5% easier than just translating the FDS game from scratch, since you wouldn't have to hunt down a translator. From a technical standpoint, the fact that an FDS game has an NES English equivalent doesn't make it any easier to hack. FDS is a totally different format from NES. An .FDS file is simply a container for multiple files, much like an .ISO or a .BIN/.CUE.
There's no guarantee that the script in the FDS ROM is stored in the same manner as the NES ROM. There's definitely no guarantee that the FDS script even contains the same scenes and meaning as the NES ROM.
It is guaranteed that you can't simply rip the script from an NES ROM and duct tape it to an FDS ROM. You're going to have to locate the script within the FDS ROM, figure out exactly how it's stored and then figure out if you can use one of the existing insertion utilities or if it will require you to code your own custom inserter.
Basically, as far as taking an NES script and putting it into an FDS ROM goes, the only thing easier than doing a from-scratch translation is that you don't have to find someone to translate the Japanese text for you. Otherwise, it's just as difficult as doing an original project. Otherwise, someone would have already translated all of the FDS games years ago.
In my experience, over the past 14 years that I've spent in the ROM hacking scene, 99% of the time when a ROM hacker takes on a project at someone else's request, it doesn't get finished, because they don't have the passion required to complete all of the necessary tedious tasks. There's a Translations Request List over at Data Crystal. If the game you have in mind isn't already on the list, you could add it, but I wouldn't hold my breath.
I've seen multiple instances of people offering money for translations, including instances in which the person provided evidence that they were capable of paying the sum. I've never once seen anyone actually complete a project, even when a large amount of money was offered. (I once offered money for an emulation related project that was posted on the front page of this very site, back when it was still thriving, and didn't have a single person take me up on the offer.)
That being said, I'm hard up enough for money right now that I'm at least willing to entertain an offer.