I never was able to find exact specs for comparison, but I think they are essentially the same.
No, you won't be able to add a diopter, but with the 60mm, you really don't need it. That's 120mm effective. It's a very zoomed it narrow view. In fact, I have to back off many times to get what I want in the frame.
I'm really a huge advocate of this port with the 9-18 & 60. So much versatility with just a lens change and leave the port alone.
I got the port for the 8mm specifically to shoot whale sharks. I've enjoyed it and used it for other things also like over & under shots.
But the last dive trip I took, I used the 9-18, 60 combo 90% of the time.