The skins would need to be remade for each car. Only a certain amount of cutting and pasting is possible, most of the cars have a shape and skin mapping that is not compatible with each other.
The XRT and the XRG are very similar though, so that one you could do quite fast. The remaining car that you want a skin for (XFG) might require a little more work. Since you only want two skins made perhaps you could have a go at it yourself?
For advice and tutorials about skinning you might like to visit
Master Skinnerz.