I see. Well I haven't been doing this myself, but I know that people use LUA to script events into missions they make. It's a quite versatile, if somewhat simply written scripting language. I know of a least a few missions people have made where you pick up soldiers and ferry them to a destination and unload them via radio commands.
If you can directly influence the cargo capacity of the Huey is unknown to me though, but I'd be surprised if it couldn't be done ;)