Having lots of fun playing around with morphing, I had a go at using 3D modellers, got the exploding part problem. Not bothered, I know that the modeller (FSDS2 of all things, I use it because it's so simple and utterly foolproof in its interface and operation) has wrecked the order of the vertices.

So, had a go with Wings3D (good idea for a modeller, amazingly stupid view controls and a manual to match). Exported a hip part, and eventually, as a test, moved 1 solitary vertex, saved and imported back into Poser. 

Everything worked as advertised until I began to turn the morph dial. While the altered vertex moved smoothly and correctly, every other vertex in the part moved smoothly as well, apparently in the direction of the face normals. 1.00 on the dial produces an enormous beach ball.

If I try import and weld identical vertices, the hip explodes. Anyone know what I should do?

Ensure that your import/export into and out of Poser/Wings does not introduce any scaling or repositioning.

Did you leave all the old morphs on when you added value to the new one?

Thank you both.

I had indeed fiddled with the scaling on import. 

I also noticed when I went throught the  "Do it again, you fool, and get it right this time"  routine that in export the 'morph target-no world transformations' box was unchecked. Checked it, went through everything again and it worked properly.
I had set the other morph to zero before, to see what the original looked like. It did not occur to me that I might be altering already altered geometry. Will remember the point in future.

"While you're here......."  (dreaded words in my job)  
is it right that the Morphing Tool only works on one part at a time? 
Is there a way to alter an area that overlaps 2 parts or groups? 
How do you select 1 vertex and move it without it jumping all over the place or moving itself to a different vertex and moving that even though all the surrounding vertices were Pinned? (is this a mouse problem; is it just my hands shake....) 

I get along with Poser Magnets, well.  It took me a few years, with Poser, to get one to work; but as I'd feared, the only problem was elementrary.  

Anywho, yes a magnet can effect more than one part at a time.  When you create magnet set you do so with a specific part selected.  To add other parts, you you go to the Magnet Zone's Properties palette, where there is a Drop Down menu of all the other parts.