Programmatically Attach 3d Mesh Objects

For a character-creation based game, what is the general process for using Unity to attach different mesh objects to a main mesh object?

Is it possible in Unity to directly modify the object at the vertex level (grow a nose, for example), once it's been imported from Maya or a similar software? How is that done? What about combining objects (i.e., average resulting from combination of two mesh)?

Finally, are there examples of movable hair?

You could make a public int or float that corresponds to the vertices in question.
this 1 i believe would be of use; Though I’m sure you will be loading your own meshdata created in another program, but you can extrapolate that if you did know the vertices in question you can “Just Grow a Nose”