Background Plant Customization?

Does anybody know how to make your own *.nl2bgp files? I'd like to make it autumn in my park, but it doesn't work out well if all of the background plants are completely green. Is it possible to convert the existing fall plants to background plants?

[Turbo]

Yes, in the environment settings you can do that.
Advanced > Environment Editor > bg. Objects

That's where you can decide what the background objects are

Thanks, Turbo. I was able to destroy the existing background plants by editing the environment file and then restarting the editor, but the new trees don't appear. Is this because there's only a single instance of each object? I tried editing one of the trees to auto-place and then adding that file to the background, but no dice.