You can edit the default objects but the cost would be that you will be seeing this change everywhere that this object is displayed. You might be able to create a new object based on the default with a different ID for the mdl and then edit that.
I had been trying to do that since I first started making scenery for my home base by using default objects, but it just did not cover my needs as I really wanted to see what I am seeing for real. This led me to start making custom 3d objects and the sketchup-modelconverterX-photoshop combination has served me well so far. I take or find pictures of the real thing, measure or guess the dimensions and after a few hours I have my object. I intend to eventually start looking at g-max or 3dsmax.
This sounds like fun too and a lot easier than the 4-wheeler. So if you have nice pictures of what you want and you can also give me the dimensions, I'll make it.