You can add drugs to the Drug menu by opening up the prescriptions of a patient and adding the drug as a custom drug. Type in the drug name and make sure that you click away from the search drop down that will appear so that nothing is selected. Then you can click on the "Custom Drug" button which will open a window with some information. Once you click OK, you will be able to fill in the details of the drug and save them.

This won't save the drug in the Drug menu however so an easy way to get around that is to just add the drug to your favourites list for easy access. You can do this by clicking on the newly created drug in the medication list. This will open a detailed window for you with a few additional options including an "Add to favorites" button.