Smacka

Yes...
Go into your "list" on the model of the X9303 that you want to buddy box and enter the "trainer" sub category. Scroll to the portion where it says "master" and "slave" for the aileron, throttle, elevator and rudder. Pick all four of them, switching them from "master" to "slave".

All four of these channels will be transferred to the buddy box, along with your high & low rates and your expos.

Note:
The "master radio (your X9303) will retain control over the dual rates and expos. You, as the "master" decide whether to have the student fly high or low rates, and as to how much expo is set for each condition. The buddy box will fly in high rates if you set the master to high rates. Same for low rates.

Without assigning the channels you want the student to control in the "trainer" section of the X9303, the buddy box will not have control over the plane/helicopter.

One last thought...make sure you DO NOT turn the on-off switch on the buddy box (your DX-6I) to the "on" position. When you plug in the JR to JR cord, the buddy box will light up on its own. If you turn the switch to the on position, you will cause the signals to not transfer correctly and you will have a mess.