That is very tricky if you also want that formation to move.
If the formation is stationary and does not move you can use navmesh cutting to cut a hole in the navmesh below those stationary agents. You can attach a navmesh cut component to them and enable it only when they are stationary. You will likely have to disable pathfinding (see the 'canMove' and 'canSearch' fields on the AIPath script for example) on those agents when the navmesh cut component is enabled to make sure they do not get confused about being outside the navmesh.
I should point out that most RTS games do in fact no do this. Some do to a degree, but many of the popular ones do not. For example in SC2 units are simply pushed out of the way when necessary.
PS: The tile size in your recast graph is very small, and will not give you much improvement over just a grid graph. I would recommend increasing it to say 64 or 128.