Hey alex,
it would be great if you could. to upload the base mesh as a *.obj (object file) first you need to turn the base mesh into a *.obj which I am sure you already know how cause your importing it into zbrush. so after its *.obj you have to zip the object up with winzip. hope ya know how to do that. if not go here (http://www.winzip.com/downwz.htm) and download winzip. then after you install winzip onto your comp you should be able to right click on the file and choose (add to archive) or something similar to that.
after you have the file zipped. come back here and when you reply it will say upload image or file. click on that and find the *zip file and and hit upload. just like you are adding your images. thats it!!!
thanks a ton alex, hope ya can get it to work!
but the way, what program are you using to rig and pose your files? xsi?