Yikes....no, I'm not sure if there's a method that does exactly what you're trying to do, however there is a workaround.
Do this:
Create 3 groups.
Your first group is all the geometry needed for your rolling car.
Your second group is all the geometry need for the transformation model.
Your third group is all the geometry needed for your flying vehicle.
Animate your car rolling for x number of frames. At the moment of transformation, animate the visibility attribute of your car group, and swap in the transformation group. Once the transformation is complete, animate the visibility of the transformation group, and swap in the flying vehicle group.
That's the method I used to animate the transformation sequence in this animation:
http://youtube.com/watch?v=6eXgTiNnpcQ
Wiring it up with a slider like you mentioned is technically possible...but it would be INCREDIBLY complicated and take FOREVER....so the workaround is probably your best bet