You could use ZB3's layers function to make a fix morph, but it's quite tiresome.
(If you use ZB2, sorry, you're out of luck)
You have to upscale and import the unposed mesh, then do the same with the posed mesh.
Then create your fix morph.
Then downscale and export everything to a modeller to extract the morphed bodypart as a morph.
(Or use Posers grouping tool)
Then use a cr2 editing program to shift the morph to the exact place at the bottom of the bodypart's node so that Poser actually reads it BEFORE it reads the joint info. (Otherwise the joint deformations would destroy your morphed shape)

To make fix morphs for joints, there's nothing better than the P7 MorphBrush tool.
Not just because the figure stays completely moveable during the whole process, but also the MorphBrush automatically creates the new morph in the correct place of the figure's cr2 so that there will be no joint deformation that messes up your morph.

Load, pose, create the morph, spawn as a new morphtarget, and just save your figure with the the newly added morph for when you use a similar pose the next time.