The folks at GSC Game World have released a fully functioning Software Development Kit for STALKER Call of Pripyat. You can edit maps using a level editor. At first the level editor is not that inviting, but after a bit of experimenting the interface will fit you like a glove. The actual interface has very fixed camera movement, you can only move in relation to other objects. This is actually rather smart for a level editor for a game consisting of large apartment buildings, because a perspective view can be easily switched to a floor-plan view. Unless you live in Europe, I wouldn’t recommend downloading it from the GSC website, several other site will have it available.