There's a mission editor that you can download and use with F11, but it's pretty much pointless since there's nothing much to place and no terrain to edit
It includes the GUI editor, which is useful if you're making client add-ons.
Actually, it is possible. If you download maps, enable them and use Mission Editor, you can add objects from that map and possibly can re-make a map of some sort.
(was it dxt? dtx?)
dif