I've got a wheel radio that I built using the electronics from a DX6i for crawling and buggys. I've just checked, and "Mix1' allows you to choose what channels you want to mix. Throttle to rudder is one of the options that you can set, along with adjustable percentages for each direction of the stick. I haven't actually checked it out on a model though.
I have used mixes on a Futaba T7. Two motors, so throttle mixed to channel 5 for the second ESC at 100% on one mix, permanently active. Second mix added 10% right motor when steering left, and 10% left motor when steering right - activated by a switch so that I can turn it off if I want to.
Works perfectly. I see no reason why the same shouldn't work with the DX6i.