I know that seems like a noob question, but I have no idea how to re-skin a prop.

I was also wondering how I could use different vehicles as props and the destroyed parts as vehicles as well.

And in Zeroeditor, when I go to the Felucia odfs and try to import "fel1_prop_fungal_patch.odf" or "fel1_prop_grass.odf" it just comes up as a blank grey octagon. Why is that? Will it show up in game?

well to re skin a prop you should probably download a program called gimp. use this tutorial here and instead of re skinning a clones you'll need to re skin whatever object you want. also check here under skinning or anything else you want to learn.

EDIT: okay honestly start with the second link the first one I just put there to give you tips to using gimp. The first link should NOT be used for beginners who don't have an idea of how to skin.
Also the second link ALSO explains how to make vehicles into props. The destroyed parts of vehicles are called "vehicle chunks" you'll have to make odfs for them, but since your new to gametoast YOU tell me what vehicle chunks you want and I'll make them for you and post the download link along with installation instructions (same for the vehicle props if the tutorial is to hard for you).As for your last question I believe that the gray odfs your putting in your map stand for the grass. If you look at the grass closely in the maps that came with battlefront 2. I believe the grass is a 2D figure. The gray blobs probably made it easier for the game designers see where there placing 2d grass. Try putting it in your map and mungeing . I don't have much experance with grass and that kind of stuff so if I'm wrong or it doesn't work than I guess I can't help you there .

One more thing, a tip for you because your a beginner i suggest you make backups of your Data_*** file every so offten. I've had many munge problems in the past that it took me hours to fix, if I would have had a back up I could have avoided these problems. Just a tip.