Okay, go to the chassis CFG, and create a new slot for the roll cage, by copying the slot line which it takes up, and putting a different number in for what slot it is. Then go to the rollcage CFG, and change slot (if it's in the mod (integrated into the RPK)), to the number you put for the roll cage slot.
If its not integrated in the RPK, then, I don't know unless I know what mods it is.