In your case I think it's totallt ok. It's more like using a tool to create 3d graphics (not that I have played or created mods for Morrowind, but I can imagine what it's like) than ripping graphics out of a game. I have thought of using that method myself, using some easy map-tool for a 3d-game and then use it as base for my game graphics.
I think it would be better though to not use the graphics as is but rather try to remove some of the 3d-feeling from it, unless you can make characters fit into it. Attempts to use both use 3d and 2d often look pretty bad.
I think it would be better though to not use the graphics as is but rather try to remove some of the 3d-feeling from it, unless you can make characters fit into it. Attempts to use both use 3d and 2d often look pretty bad.