Dropping in here, as I've a similar question and an addition to add
Such has definitely been possible at one point, this was done with JNGP (Jass Newgen pack/editor) it had a specific function to add more tiles than normal, how it does this exactly is beyond me but it saves the map after selection through it's own menu, implying it having to save the tile data in it's own way to surpass the limit, however, JNGP is a tad out of date so if you want to use this trick in newer maps, make the map, grab the tiles you want, save, then use a newer editor, as long as you never try to edit the tileset again, it will keep all of your fancy tiles.
But trying to open the newer maps in old JNGP doesn't seem to agree well, so beware, so I too look forward to when say, sharpcrafts gets this.
Per Example of the menu and tiles
You can do this
I realize my post is slightly redundant, but you can do what I did and make your tile selection here first and use newer editors to do the rest, moreover, you can be snarky and use only grass/buildable tiles and override their textures with whatever you want later and have a fully pathable map.