Transparent Textures

I'm pretty good at scenery design, and I've posted a few airport packages, but one thing I've never been able to figure out is how to create semi-transparent textures. Say, for example, I want to create a simple fence. I want you to be able to see through it, but i don't want the metal itself to be transparent. How can one do this without using an Opacity Map in GMAX, which won't carry over to FS? I've seen it done before, so I know its possible, but I don't know how. Does it have something to do with getting an alpha channel in a bmp?