Usually you can add hens to a pair ,if you put all birds into a new pen.If you add a hen to an existing pen,the birds that are in their will defend their pen to any introders.Yes, even if it's another hen.But the problems is,your talking about chukars.They are very mean little buggers and you cannot trust them one bit.If you do add a new hen and they seem to get along fine,keep an eye on them all the time.A 6x2 pen is not very big ecspecialy for chukars.They need space to get away from each other when trouble starts.Is their pen on the ground?Or on wire?I had great luck with raising them on the ground with masonry sand on the floor.Give them lots of hiding places and a couple of roosting places.A single pair of chukars will produce a lot of chicks.Maybe you should get another pair and run 2 breeder pens.Then you can use chicks from each breeding pair to start unrelated pairs for the following season.Put the hens from one pair to the males from the other and vise versa.Or even trios or quads.
In N.H.,Tony.
