I will take a stab at this to see if I know what I am talking about.

The fish will spread out into the new area and balance themsleves out. That may take a few years for that to occur. If you want immediate results, then I would say figure your additional area and stock accordingly. If you add 1 1\2 acres then stock the bass and bluegill that is recommended for 1 1\2 acres.

My brother-in-laws family had a sandpit. They stocked it when they first started. As they would increase the size of the pit, the fish followed. They didn't decrease in numbers in the old part of the pit, they just increased to fill in.