Creating a new mesh from part of another mesh provides some interesting difficulties:

1. Creating a new mesh is as easy as var mesh = new Mesh();.

2. Another task is getting the vertex arrays from the first mesh. If you are simply doing a distance check, then it isn't very hard. If you want complex shapes, then you will have to to find an algorithm that isolates only the vertices you want- maybe use vertex color.

3. Recreating the triangle arrays is probably going to be the hardest part. If you are lucky and removing a submesh, that will save you some time. Otherwise. Same as above. You will need to procedurally recreate the triangle indices.

Decombining a mesh won't come easily. Your best option would be to store the vertex and triangle indices before you combine the mesh so that you can take it apart more easily.