Yes, this is possible in Encore. Basically what you need to do is set the buttons up to auto-activate--that is, you simply roll over them to activate their link instead of clicking. This is done by selecting a button and checking the "Auto Activate" checkbox in the Properties palate. Once that is done for all of your buttons, link each one to a separate copy of the menu you're on. Each of those copies of the menu should be exactly the same as the original menu, but with the proper thumbnail video inserted as the motion menu background.
The page won't load for me right now, but Steven Gotz's link page (http://www.stevengotz.com/tutorials.htm) may have the explanation as to why:
Free Adobe Encore tutorials. He is a wee bit Scottish, but you can understand him.He sometimes has bandwidth issues. If you don't get his site to come up, wait until after Midnight EST for the allottment to reset.