In my experience, you can either add the artciles via the gui in SSMS or use the system stored procedures to add the article(s) to the publication
Once added this will invalidate your snapshot and also it will not replicate the new articles in the publication.
Run the snapshop and reinitialse the subscriptions via replication monitor or using the SQL Agent Jobs \ system stored procedures.
MCITP SQL 2005, MCSA SQL 2012