Changing the dictionary used by OS X Dictation
OS X Dictation will let you dictate people in your Contacts. Make sure any words you wish to say are included in Contacts (you can place multiple words in the first name or last name fields) and providing you have enabled Enhanced Dictation the words should be available immediately. When dictating, if an alternative word is available, the word will be underlined in blue and selecting an alternate word will, over time, allow it to learn the word as a preferred dictation result. I have not tested deleting a contact and seeing whether the word would remain in the dictation repertoire.