How to expand my irrigation system
I'm going to refer to the valve at the top of your picture, but the same would apply to the bottom.  Between the last tee and the final ell, there is enough pipe to cut off the ell and glue on a coupling.  However, there is not enough room to glue on a coupling and a new tee to leave the top valve in its current position.  That top valve is going to have to move.  That would be a lot of trouble.

Instead, I think you should dig to the right of the picture and tap into the main line coming in to this current box.  There is bound to be a straight section of pipe that you can tee off of using a few unions or a repair coupling.  Once you tee off that main line, you can place a new valve in a smaller box wherever it's convenient.  Running your spare wire to the new box should be pretty easy as well.