I believe that's possible but correct me if I'm wrong. You can do that in the editor with the childs editor. I once placed an crane arm on a truck, looked funny, didnt function. But it's not so hard to do. Load the mod in the editor, select the vehicle and press 'edit'. Then their should be a menu to edit the childs and there you can find the water cannon and place it on the truck. Hope this is any help.