If you need to add a local term to an open vocabulary:
The next time you use this vocabulary, your local term will be present in the list of choices.
If you want to use a term without adding it to a vocabulary:
See Main Menu | Edit Menu | Vocabulary Editor
Note: that if the vocabulary is defined as an 'open' vocabulary, you can add new terms simply by typing them into the appropriate element during editing. Follow this link for more details about Open vocabularies