Object menu -> Create magnet (with the body part you want the magnet applied to selected of course)

manipulate the magnet as needed.

Then,

Object menu -> Spawn Morph Target; name the morph

Or, if using multiple magnets on multiple body parts and you want one dial for all of it,

Figure menu -> Spawn Full Body Morph; name the morph

Edit: For P9/10 (and earlier), you'll have to create multiple magnets on multiple body parts, if the morph affects multiple body parts.

In PP2014 magnets can be weightmapped, which means you can use less of them.

Have not used it myself, haven't had a reason to, but that's the theory at least.