Assuming you are using easyC, could control it with tank drive by placing the "tank four motor block" and adding two "Motor Module RX" blocks for the fifth and sixth motors. All you have to do is make sure you assign one to the left and one to the right joystick.
Although I have no robot to test this idea on with me, one could possibly make a six-wheel arcade work by using the "arcade four motor" AND an "arcade 2 motor" in the same program. If you feel luck, maybe you could give it a try
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Oh, and of course, make sure its all in a while loop (I've made that mistake about 50 times).