Daz and Poser models are broken into individual parts for rigging, which is why you’re seeing the seams when they are exported or you open the .obj file containing the figure’s geometry. As Aurick suggested, you can tell ZB to weld everything together when ZB loads the object, BUT you will then be faced with how to split the object back into individual pieces to make morphs out of it. DAZ has come up with a solution for this in ther Figure Setup Tools sets.
To export a figure of mulitple parts (like chest, right arm, etc.) so you can morph them, you will want to be using Morphloader Pro (part of Advanced Figure Setup Tools.) You will then export from Daz Studio using “bone welds” (which join the parts together) before sending to ZBrush so you have a single mesh. Morphloader Pro will then take the Zbrush morph and split it back up into individual parts like Daz Studio/Poser expects.
http://www.daz3d.com/i/shop/itemdetails?item=8114
http://www.youtube.com/watch?v=jfA803Pqc08
Without Morphloader Pro, you will likely need to work on morphs one part at a time (like “head”).
-K