Horizon sells a buddy cord that allows a Futaba TX to be used as a buddy for a JR master TX, but not the other way around. However, the whole setup is a bad arragement, and when I found one at the LHS and read the instructions, it proved that the coversion cord was basically useless.

Since JR buddy boxes use their internal batteries, JR trainer cords do not provide power like the Futaba ones do. So, if you have a Futaba buddy box that doesn't have batteries in it, you can't use it at all.

And it gets worse from there. In a JR box, when you plug in the trainer cord, the TX powers up. It's NOT transmitting, but it's "active", the lights light up, screens come on, and you can do programming on computer radios, etc. (this is because of the DSC feature, btw). Futaba boxes don't do this. So, the Futaba box has to turned on with the power switch. Which means you have to pull the crystal or module to prevent it from transmitting. Very annoying.

Using a Futaba master with a JR buddy is apparently easy. I haven't seen it done in person, but apparently you just need to do a quick and easy cable hack to make a cord that works. I plan on doing it myself, but haven't gotten around to it.