You can add more mah... but it adds weight and eventually you get to a point that makes the plane too heavy to do what you want to do.
I commonly have planes carry double the mah recommended. Almost any airplane will fly with the added weight. But I don't attempt any high G flying with the added weight without ensuring the aircraft is capable of handling the added stresses involved. Extreme low speed aerobatics (commonly called "3D") will also suffer from the added weight.
How much extra you can carry varies greatly with what plane you are flying. Some planes can reasonably carry more than double their "ready to fly" weight as cargo or added battery capacity. You won't be doing any aerobatics with something loaded that heavy if you want to keep your wings attached though.