A shipping container uses the full depth of the corrugated metal walls to span 40 feet (much like a wide flange beam needs it's web. It acts as a deep beam which is as deep as the height of the container. Therefore, structurally, these walls are very important to keep. To keep costs down, you want to "let the container be a container" and try to keep the walls. Unfortunately that is often at odds with flexible space and leasing requirements of offices being a minimum of two containers wide. So some walls need to go. If the walls are fully or partially removed, the container becomes much more complicated. It can lose almost all of its stiffness and shear capacity. You need to introduce reinforcing (horizontal members or columns) because the small base channel cannot span very far in flexure. You can accommodate small window openings but determining size for unreinforced or minimally reinforced openings can be complicated. This is due to the walls having different properties in different directions - in the horizontal direction, the web is as flexible as an accordion - but in the vertical and shear orientation it is very stiff. There is additional complexity in determining how to economically reinforce modified containers - that is, how to minimize field welding and simplify the connections to adjoining containers and foundations.
The container can be joined the same way they are stacked on a container ship, using steel assemblies that have manual levers to lock the containers together at each of the four corner castings. They may also need additional welded plates to tie them together horizontally. Given all the costly reinforcing required, the only way to make a shipping container building economical is very close collaboration with the owner, architect, and builder. This was the case on the Box Office project.