Most people that have the cash use ZBrush, you can use just about any modeler to create morphs. Helps to use one that is a little more organic modeling friendly, Something with brush tools... With ZBrush there is GoZ which imports your morph work into poser without alot of scaling issues that you would encounter prior. I'd say the biggest path to Poser problem with external morphs is the comparitively tiny native scale of Poser objects, these are things you will have to overcome variously for the different modelers you might use.

If you can tolerate Blender that's a good one. I use an older copy of ZBrush that will run on my machine in conjunction with more utilitarian adjustments in Wings. Hexagon also is pretty good for morphing but doesn't work so well on my Mac, alot of people use Hex for Poser stuff. There are others of course. Sculptris? People use that... really anything with brush deformation tools would be the path of least resistance.

Don't discount Posers own morph putty tool and magnets, you can actually do alot of things with those too.