First the quilt is gorgeous!! Since it looks like you only have enough to go down the sides, what if you broke up the blocks and make side corners with them. Any left over you could sprinkle in the sides. Like a pieced border. That way you could expand it and use one of the other colors as a framing border and then do the pieced then end with a larger border. Say put a 1" green frame border, the the pieced border with the blue and finish with the purple for the outside border. :-D
Or are you just wanting to add length? If so, then the picture above is a great example.