you can add a sump to an existing tank with np.
the overflow box should be about 5x - 10x the size of your tank + sump. so if your tank is 26g and sump is 10g = 36g, you should have 180 gph
or above, at least thats what others have been saying.
the return pump would have to match your overflow box AFTER you consider the head loss (loss from being pumped up against gravity).