You can increase the length of a tween span without affecting (stretching) the animation by holding Shift and dragging the tween span's end.
Also, like classic tweens, you can put blank or static keyframes after a tween span if you wish (right-click to insert a blank keyframe, then you can copy and paste-in-place the last frame of your tween if you want a static frame there).
For other such keyboard modifiers, you can see this post: http://flashthusiast.com/2008/11/24/lists-of-keyboard-modifiers-in-the-flash-cs4-motion-mo del-lets-you-do-more-stuff/
Didn't work. I have a new motion tween preset (the light blue) and tryed to had a frame or drag out the lenth of the time line and all of the keys and easing break the moment the time line is adjusted on the tween. Try it out for your self you will see what I'm talking about. Look over the attached FLA.
I've done the Shift-drag a million times, so I'll need to see what's happening in order to help out. Can you try re-attaching the FLA file or describing exactly the steps you are taking? I don't see the FLA above. Or you can send it to me at jdh (at) adobe if the forums aren't working.
Remember that eases apply across an entire tween span, so in that case it's expected that the ease will stretch while the other keyframes stay put. The ease for the animation will need to be adjusted to end at the end of the position (etc) keyframes, or you'll want to split the span instead of stretching it.