You cannot add cars, locomotives or anything else in Surveyor. Nor can you change the smallest detail in Surveyor as that will modify the session and nobody else will be able to join it. Nor will you be able to join another player,s session. What you can do is start a session and add consists via "Quickdrive" while nobody else is in the session. The only other course of action is to create your own session but you must have the Author,s permission before uploading it. As an alternative you could ask the author to add the locomotives or Cars you would like in his next version.