Hi Leonard, it's a little bit hard to tell what your model is just from a text description, if you could please post a file attachment with your actual model in it that would help to clarify it a lot.

But from what I can tell from your description, you will probably need to do a boolean operation to cut your solid into 3 different pieces, then move the outer pieces to relocate them and scale the inner piece to fill up the gap, then union the parts back together again.

Please see this previous post for a demonstration:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2019.3

Does that handle what you need?

In the future I would like to have a kind of "stretch" function that would handle this more easily, but that technique above is the way you can accomplish it currently.

- Michael