Is it possible to add a TT shifter to Di2 junction box?
Yes, of course, that's possible. You just plug the cables into the brifters. Peel the grip-covers forward and you'll see unused ports underneath. Those may be used for sprinter buttons or for climber buttons as well.

But I'd suggest you replace a triple box by a quintuple box (Shimano SM-EW90-B 5 Port Di2 Junction A) to make the connection easier without undoing and replacing the bartape. Another advantage of the quintuple box is the fact that you can remove the aero extensions without fuss.