Oh, if you're not starting with a Poser head, then it's different.

What I do in that situation:

1. I model the original item, then export it.
2. With the original still sitting in the modelling app, I fiddle with vertices,
   then export the changed version.
3. I bring the original item into Poser, with all boxes unchecked.
4. I then hit the 'Add morph' button on the Poser parameter dialog and pick
   the changed version.

This works even if there is a scale applied to the export, because the
original and changed versions are both on the same scale.

However, if you've turned the original into part of a Poser figure between
steps 1 and 2, then this probably won't work.

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