You don’t have to use Zbrush. Blender works just fine (and it’s free).
You can also use Hexagon (another old DAZ product that hasn’t been updated in years but still works for the most part). But Zbrush and Blender and Hexagon are by far the most popular.
Key thing to remember is export the mesh as the Base resolution with no subdivs and as an OBJ and whatever you do don’t alter the vertices. You can push and pull the vertices all you want but you can’t add any new one’s and you can’t delete any.
Then it’s just a matter of saving out your modified figure as an OBJ and load it up in Morph Loader Pro.
There are other things you can do to hide limbs and other parts of the body but that’s the very basic gist of things.
I’d try practicing exporting your figure as the Base resolution with no subdivisions and maybe importing it into Blender and practicing morphing the face and importing that back into DAZ and using Morph Loader Pro to get the feel for the workflow. Then it shouldn’t be too hard to take what you learn there and apply it to the rest of the body.
This also works for applying your own morphs to clothing too for example. For example say you want to add a more stylized package/ bulge to a male pair of underwear. The same exact principles apply.
Note you can also use the Mesh Grabber script to modify the figure’s mesh within DAZ Studio and (I can’t remember if this is built-in or an add-on) but there is a way to save your modifications out as a morph preset too. So that’s an option too.
Then once you’ve amassed a library of morphs that you’ve created it’s really just a matter of saving them out as a Morph Asset (Support Asset) and package them all up accordingly.