But they do cut into the hull plates, no?
I know that in other maritime construction -- cruise ships for example -- ships have been lengthened by adding a section. Don't mean to compare a nuke sub to a cruise ship, but merely that non-nuke shipyards have done this before.
Yes, they do that. As a matter of fact a submarine in overhaul gets quite a few hull cuts because there are plenty of pieces of equipment that are just too big to unship though a 31" hole.
There was at least one boomer that I know of that was converted into a prototype. Basically, a floating training platform. The missile deck was cut out and the forward and aft sections welded back together. Kinda neat.