The wiring diagram does show the onboard switch on the hot side of the bulb. It may be possible to move the switch to the ground side of the bulb, which would mean the bulb socket would have to be isolated from ground. Then the door switches could be connected in parallel to the onboard switch. And any switch would provide a ground and turn the light on.
If you somehow isolated the dome light body from ground, then the onboard switch wouldn't work. I think that might be harder to do than isolating just the bulb socket.
Otherwise, leaving the onboard switch as-is, you'd need isolated door switches with two contacts, one from power and the other to the bulb side of the onboard switch. That would mean a switched power wire from each door switch to the dome light to bypass the onboard switch (a total of 3 wires run to the light, one constantly powered to the onboard switch and one each from the door switches.)
