When you want to move a stack/unit between structures in different regions, you must first drag the stack/unit outside the structure. Only then can you click on the special order to "enter a structure" either before or after you drag it to the structure in the other region.
Perhaps, whenever a stack/unit enters the state of having a move pending, then "enter a structure" special order should become available. Might be a little more intuitive than the player having to realize that this is a three step process as it is now.
Thanks.