I haven't tried, but I don't think you would get too much more flight time, and the extra stress on the bird couldn't be good either.
The P2V+ without the battery is 2 lbs. The battery is about 13oz.
Adding the battery would probably put the drone to the upper part of the weight limit... the motors would race to keep the thing in the air. I'm not sure what the smart features of the battery would do either... one might decide to turn itself anyway. You could remove the camera and strap it underneath, but I think it would work so hard to fly the extra flight time would be minimal.