Hi Bill,
You can add additional drivers by editing the session. In the Session editor, which opens up Surveyor, choose the icon that looks like a piece of paper. This is located on the top bar. Once this loads, click on the line with the engineer's image on it, and you'll see your consists which are assigned to drivers. Click on add driver and you can then assign them to other consists.
Now keep this in mind. The more drivers you have running around, the more resources will be used causing Trainz to get slower and slower. I found that about 20 drivers can be the cusp of the tipping point for this. Having said this, I recommend using the Quick Drive rule. You can have as many consists sitting around and all you need to do then is assign a driver to one of them when the time comes to drive.
John