IMO, it's in your hens' best interest to give them as much space as possible, so adding more birds to an already small coop wouldn't do that. Could you do it? Sure, but just be prepared for the possible (not certain) consequences in behavorial challenges that often come with cramped spaces.
I'm looking at your area - don't you guys get decent snows??? Maybe I'm wrong on that though...but if you do get decent snows, know that chickens do not like going out into snow, so they will most likely spend a lot of time indoors at those times. Winter time is when most people see behavioral issues w/their birds due to overcrowding. Folks in year round mild weather areas don't have those issues nearly as much, because their birds are outdoors year-round. If your run is roofed to keep snow out, that will help.
Good luck with what you decide.