I have always had problems like this with ZBrush and Poser. If it is the same as I encounter (may not be, and very well may be the one mentioned by pjz99) Zbrush has a problem with Posers small scalle. I use objaction scaler and scale the obj up by 500 befor I take it into ZBrush.  Then once I am done in ZBrush I scale it back down. Also use the geometry from the geometries folder if at all possible.