You probably would need wood under the shelves, or else they will droop. I used PVC piping instead of wood.
The best way to zip tye the NIC panels together is to form an X when zip tying. You overlap the zip tyes. It is kind of hard to explain.
Um… The only Idea I can think of to extend your pen would be to just make a condo out of the NIC panels (if you have enough cubes) and attach your pen to the condo – that is my setup. Or you could just make your NIC panels into a Pen like thing and just make a bigger pen ( I like the first Idea better though).
I don’t know if this would work but you could make part of your pen into a rectangle (With an opening on the side) and attach NIC panels to the bottom to keep it in the shape of a rectangle. and then you could add shelves to that as if it were a condo skeleton! Sorry if my explaining does not make sense, but just experiment with things. The beauty of NIC cubes is that you can take them apart and rearrange if needed! just be sure to buy a bag of 1000 zip tyes – I used more than that for my cage (only like 30 more though).
Good luck!