You could modify with longer arms and such. Stability likely would not be a problem since most FC's have more built in flexibility that most realize. To play with changes in payloads you run into compatibility with system firm and software. If the new payload does not use what Yuneec uses you're pretty much dead in the water unless you added a control system for the payload. Now you dealing with the additional weight of the payload and payload control system plus the weight added by extending the arms and wiring, so flight time will be reduced. You could use a larger battery but those add even more weight so much of the additional battery capacity is wasted through the system having to produce more RPM/power to lift the extra weight. The motors themselves are relatively high kV and unless the propeller size is increased (to 11-4's probably) they really aren't capable of lifting much more than they already do. Of course if you change prop size you'll have to develop a mount for them to fit the existing motors. APC makes pretty effective 11-4 multirotor props. To get more out of the motors and make effective use of the props you would need to increase the cell count of the battery, from 4s to 5s or 6s, and I don't know the existing ESC's are rated for any more than 4s. If they are not you would burn out the ESC's as soon as you armed the motors and started to spin them with the higher voltage battery. Has this become a lot more complicated than you expected yet?
Candidly, if you want to try this kind of stuff pick up a Tarot 680 frame, a Pixhawk or X2 FC, and initiate some research to select props, motors, and ESC's based upon the weight you want to carry. From there research gimbals and gimbal controllers suitable for the types of payload you want to use. It's not as simple as just tossing longer arms and a few other hard parts at an existing consumer drone airframe. Modding such an airframe will end up a pretty expensive experiment, one likely destined to failure. But if you have money to burn....
