No resistors needed! (Just discipline). Putting batteries in direct parallel is valid, as long as the charge state of both batteries is the same. In practicality, both fully charged is the only valid way to achieve this. The discipline part is to always connect two fully charged cells together. If you don't the cell charged to a higher voltage will 'charge' the other cell, and for cells in series, that is not good, as the voltage could potentially go beyond what is stable for the chemistry involved. (A detailed subject of it's own).
I am eventually planning this to add a 1000mAh LiPo to my Q500 via a JST connector out the top. It will be an optional payload so I dont always have to fly with it. It will take some time to pull the frame apart to add in the connector as they have the main input connector buried deep inside the quad.
Not sure about 10-15 minutes longer, as the extra weight takes extra energy to lift!
I just devised a way to launch the quad without it's landing gear (at the peril of my camera, I might add). This save more than enough weight to add the 1Ah battery to the mix. It also provides some redundancy in case of battery failure. If the battery fails, the quad comes down like a brick. Bad thing about quads over fixed wing IMO.