First of all ... what LiPo's are they .
Phantom 3 uses 4S High Voltage LiPo's .. you do not say if HV version or not. If they are standard LiPo of 4.2V max per cell - then you would have serious risk if you series connected with the existing battery as in the 'piggy back' system on Youtube. The DJI packs are 4.35V per cell full charged.
Your packs are 7.4V which means 2S ... therefore you would have to series connect to get 4S ... but again - if not High Voltage versions - you should not use with the Phantom.
Finally - adding extra battery is also subject to the extra weight to lift and the increased RPM on motors to achieve lift. Two things result :
1. Flight time is not increased as a linear amount based on increased battery capacity.
2. Motors work harder to lift the whole. This means reduced max speed and performance.
My honest opinion - with those 2S batterys in series - you will only add 2700mAh to the capacity for a significant weight increase. But of course if not HV versions - you should not use them anyway.
Here's a P4 video :