You can't manually enter an address while driving, but you can give it a voice command to do this. I was just playing around with this last night. I couldn't remember the specific commands to use to enter an navigation address. Then I realized, when you hit the voice command button on the steering wheel, the screen does give you a number of command suggestions which is helpful.
I think it ended up being "SET DESTINATION" which was followed by the system asking me whether I wanted to use an Address, Previous Destination, etc. I tell it "ADDRESS", then it prompts me to tell it the complete address including City and State. System then asks if the address it understood is correct and will then set it as the Nav destination.
I think I may copy and laminate the list of voice commands from the manual and keep it as a small reference card in the truck.