what version of Poser are you using?

If you have Pro 2012 or 2014/game dev, then you have a built-in Poser to Z-Brush utility called GoZ. It can be found at the bottom of the figure menu in Poser. "Export figure mesh to GoZ"

Just select the figure you want to send to z-brush and click that option in the figure menu. It's also at the bottom of the Object menu if you are working with a model that isn't rigged.

A menu will come up with a few options in it. If you want to send the figure in a specific pose, then check the box next to "Export Posed". Otherwise leave it unchecked (default) if you want the figure to go in its zero pose. (Generally you'd only export posed if you are creating JCMs).

Material is on by default. This tells Z-brush to create polygroups based on the material groups set up in the figure - useful for texturing in Z-brush. If you select by body part, then it will create polygroups based on each body part.

Once you make the changes to the figure you can send it back to Poser and everything should work fine.

If you're just using the figure as a base to build clothing around, then you can send the clothing mesh back to Poser via Z-Brush's GoZ menu. Just make sure you have Poser selected as the active GoZ app.

If you're creating texture maps, then you'll need to export them manually from Z-Brush and set them up in the material room yourself.

You can divide the mesh however many times you want. Z-Brush will retain the vert order and send the lowest res mesh back to Poser without breaking anything. The only drawback is that Poser's SubD won't use any of the sculpting you do in z-brush. But you can divide the mesh to create morphs if the base res isn't high enough for you to work with, and the low-res version of the morph will still work in Poser. (At least that is the case in 2014/GD. Haven't tested that in 2012).

*Should also note - if you close Poser or Z-brush, before sending the mesh back then the link between the two apps is broken and Poser won't recognize the returning mesh as an altered version of that figure, so it will bring the mesh back in as a static mesh - no rigging, etc. So don't close either app or change anything in your Poser scene until you're done.

GoZ isn't available in the standard versions of Poser 9 or 10. You'll need to import the figure into Z-Brush manually if you're using one of those versions.