Can I take adult ADD / ADHD medications only when I need them?

No one can pay attention to something they are not interested in. You don't have ADD. However, if you are willing to take medication to be less bored, that is a different story. In either case, the medication does not build up in your system, i.e. you can take it any day you feel you need help. People who actually have ADD need this help every day, which is why the drug will not likely be prescribed to you.