Nope, you can only create sprites, monsters, and title screens. Sprites can be used to create new scenery for your map (such as a TV), but you're stuck with the 24(?) backgrounds it already gives you.

Yeah, it is possible to make it so you can walk on them, but it's with restrictions. There is no "passable" option, you'd have to manually tell the party to walk onto the "tile" with a movement command. Along with that, it would almost have to be a tile that didn't move around on its own. If it's not moving at all, walking onto it is a simple as using a move-to-spot command. If it is moving, it gets more complicated, and I'd rather not get into the details of that unless its necessary.

One thing to note though, is that if you do make it so you walk onto the sprite, if you try to go over the top of the sprite, the player will appear to step under it before appearing on top. It's not a pretty sight, but it's doable if you really want a custom floor tile. Also of note, is that since it would have to be a sprite, you can't put too many of them on a map since they'd have to be events.