There are various ways.
1. Runway12 Object placer which is freeware.
2. Build the object yourself and place it using .xml (not sure on this one)
3. EZ Scenery by Abacus Publishing.
4. GMAX - see Robins Windowlight website. Great article on there.
5. The SDK's from Microsoft
There's bound to be others.
I use EZ Scenery and if you go to most download sites under the Scenery Creation area you'll see the various object libraries that can be downloaded which give you the objects and their textures. You then place them using in my case EZ Scenery.
The upcoming Friday night FS9 Missions have been created in this way.
In the case of EZ. Anybody can download the Library for free
here. (it's about 8mb for the entire thing). You just need someone with EZ to create the scenery .bgl with the placement and object instructions. You then place this in your Addon Scenery/scenery folder and away you go.
If no one else steps up. If you want to flick me a PM with what you're after, I should be able to put something together for you.
Ian
oops sorry forgot. Some addon scenery like Robins excellent RealNZ series comes with vehicles.