Free up space for monster sprites?

With ASM, you can pretty much expand anything. I'm not very familiar with the code for this, but you can probably move all of the monster sprite data to a new location in an expanded ROM, then move the pointers or part of the code that tells the game where to look for monster sprites to that new location. If you, for any reason, go over 255 sprites, you will probably have to do some additional hacking to make the spriteID 16 bit instead of 8.

And lastly, in order to make sure you are inputting new monster sprites into the correct location, make sure you know how long each sprite is supposed to be. You can do this by importing a sprite in an unhacked game and comparing the differences in a hex editor, then copy the edited code into the hacked game at the new location, at the end of the list to make the next entry.

It will definitely be a good amount of work to do manually, but if you were hoping to use editors only then I'm afraid its not possible. ASM will definitely be required.