That isn't something I've done before myself, but I know it is possible.
Besides the UI for loading/selecting them, the trickiest part would probably be getting the "eyeshots" right. I manually choose what part of this is clipped for each sprite, it isn't an automated process. So new ones imported in would either have to have the player include an eyeshot or a UI to let the player indicate what part of the sprite should be used as an eyeshot. (I can't think of a reasonable way to automate such a thing.)
There are some extra restrictions that would come in for monsters, since their size is a factor in combat, but I don't think that would be the rough part: I'd either communicate that to players so they can follow it and/or have the program handle "incorrect" sized images (though this might result in some frowny results.)
The walls, doors, and other dungeon bits would probably be relatively straightforward at least.
Anyway, I'm not unwilling to do it, but it would be a bit of an undertaking I suspect (particularly the UI for it.) After the current build is out, I'll ask in the Discord and see what the general interest level is for this among the current players. I can't promise I'll do it immediately even if it is high (I already had the next update pretty planned out), but it can definitely get on the "sooner rather than later" list.
Thank you for the suggestion. :D