You can expand enclosures using so-called enclosure expansions. You can find them in the shop right with the regular enclosures:
These can then be placed next to an existing enclosure - you can move it around a bit until it's expanding the enclosure in the way that you want.
If you have two of the same enclosures nearby, it might try to attach to the wrong one - so make sure it's where you want it to be, then confirm the placement.