In a recent IDW Star Trek comic [I, Enterprise PtII][1], we learn that not only can the saucer section of the (Kelvin) Enterprise be separated but that it can also be *reattached* if the separation sequence hasn't yet been completed.

![enter image description here][2]

We actually see this in Star Trek: Beyond

https://www.youtube.com/watch?v=W7b4MneaHIo


  [1]: http://en.memory-alpha.org/wiki/I,_Enterprise!_Part_2
  [2]: https://i.sstatic.net/YuyOa.png