Can you put tandem breakers in a panel box that is full?

It depends on the type pf panel. Some panels allow for half sized tandem breakers and some do not. It all depends on the busbar in the panel in relation to the part of the breaker that snaps/bolts on the bus. My advice would be to try it and if the breaker will snap/bolt on the bus you are good to go.... even better yet ...call an electrician...we work hard to learn the methods and codes in order to provide safe electricity in homes and buisnesses and for someone to just go online and try to do something themselves in order to save a few bucks is not cool and really just downright dangerous if you dont know what you are doing.