A power bank small enough to fit on top of the Spark wouldnt be able to supply power fast enough. For that matter a large power bank still charges at a rate much slower then would be needed to fly.
The Spark fly with a power consumption of 60W for about 12 minutes.
Charging through USB port (in theory as in practice it is not possible to fly and charge at same time) is max 5V x 2 A = 10 W with an estimated efficiency of 90%, it is 9W that can be provided.
So the extra pack can give only 9/60 0.15 or 15% of the energy consumed. So in best case, it would extend the fly time by 0.15 x 12 = 1.8 minutes for a total of ~14 minutes.
This, if the battery had no weight. With the added weight, it is even possible that it would finally reduce the flight time.
I don't even think to the weight unbalance it would create and I doubt the Spark would take off.
So it is a dead idea from the beginning.