Yeah, I tried that one time but gave up on smoothing the tunnel.
The only thing I could think of was to do this:
But that's really just exactly what Snarkpit tells you to do. And this method only works with 2 pieces; any more and you get a plethora of errors.
Unfortunately this also results in the rounded part being thinner than the rest of the tunnel, and you can't really move the vertices around lest you risk building invalid structures.