I don't really understand ZBrush, but I suppose it's possible to use that.. Most of my experience is in C4D, so I'm not sure exactly how well that translates into trueSpace. I'm guessing trueSpace supports organic modelling and soft selections (i.e., if you select one polygon and move it, it pulls the surrounding ones out with it). That would probably be the easiest way to do it.

Export the tear prop as an obj, and then export the head of your character as an obj. Import both into trueSpace, making sure they're seperate objects. Then simply keep pushing polygons until they line up with the characters eyes. Delete the head, and then re-export the modified tear shape as an obj. Import back into poser and simply re-add it to the library like I mentioned above. I know that workflow would work in C4D, not sure about trueSpace..