I don't know how you've been doing it so far, but the morphing tool creates a morph dial called Custom_Morph, which you can use to animate the morph - so you can go from a setting of 0 at frame 1 to whatever you'd like at frame [end] :)

So in your example. Make the morph at frame 1 and set it to 0. Then, at frame 10, set it to 1 and voilá :)

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.