Modability of unity 3D engine games

Is it possible to mod my game or make users add content like new scenes?
I come from a leveldesign/modding background and would love to support this kind of stuff to my game,
but since Unity builds and compresses the game it seems impossible to add modding support?

Not to mention the big steam updates for a small change made to the game.

Not sure if developers read this board, but I think its soimething they would have to look in too,
when I started my game Unity was the only affordable choice, but if I had to make the choice again today it would be different for this reason.