Hi Mike,

> But for some reason, the new surface refused to Join to the existing structure.

You've got to remove the existing bad surfaces first - the area that looks like holes actually has some surfaces there, but they have a mangled trimming boundary so their display is messed up.

So to fix that you've got to do an Edit > Separate first to break the thing into individual surfaces, then do a window select (with the "inside only mode" - start in the upper left corner and drag to the right) to capture the bad faces and delete them. Then you'll have an actual open space there that can have something joined in.

Otherwise if you don't delete the bad ones the edges there are already joined between 2 surfaces and that's why they would not accept any new joins in there because if they did join in that would be trying to have edges shared by 3 surfaces instead of just 2 surfaces.

- Michael