You can do it once anyway.
I've heard stories about people doing this WITHOUT a reducer, where the smaller shell falls down into the barrel, then a larger, full size shell gets inserted right after it and fired. I hear it's real ugly when that happens.
Oh it is. I saw it happen once to a very, very nice Browning O/U. Why on Earth my idiot boss had a 20 gauge shell in his pocket I'll never know.
Why he didn't notice that it was a different color, bright yellow instead of red, I'll never know.
What I do know is that doing it will completely destroy a very nice shotgun along with a significant portion of the person firing it.