I'm uncertain about in Greenland but... in the USA the DX6i is a radio made for AIR use only... We also have radios that are made for Ground use only. This is a restriction put on us by the governmental FCC agency that has Air/Ground vehicle radio's are required to use different frequencies to avoid them having radio frequency conflicts. So here it would be illegal to do what you said with that specific model radio.
Having said that... and assuming it isn't illegal there or you have a radio that is made for ground use. You would first need two electric motors capable of forward/reverse, you would also need two ESC's also capable of F/R. Then just plug the ESC's into the channels that are assigned to the sticks where you want to use them. I would assume the ESC's would need their center calibrated for the zero throttle spot for each, so you'd read how in the ESC's manual since that is likely different for each brand ESC. On the transmitter, you would probably have options to have the sticks spring back to center, have smooth travel or be ratcheted... again each brand will differ in how to set these and I'd have to refer you to the manual for the radio for that. The final step would be for one or the other motor channel, you would need to reverse the channel so both motors will have the same F or R travel... exception, on brushless motors you can just switch any two of the 3 leads to accomplish the same thing.
So first step is to determine the laws where you are and if you have restrictions on what type of radio you can use or not. The next steps are choosing and sourcing the motors you need for the size vehicle you have and the same for the ESC's. I know these types of motors and ESC's exist since I've read about their use on 3D airplanes and I'd assume the same is true for ground craft also.