How to add a Japanese name that does not appear in the Kotoeri registry? (OS X 10.11)
Edit and use your user dictionaries (For OS X 10.11 El Capitan (and 10.10?))


  
  While using a Japanese input mode, choose Edit Text Substitutions from the Input menu in the menu bar.
  In the Text pane of Keyboard preferences, click the Add button .
  In the Replace field, type the hiragana reading of the word (up to 32 characters).
  
  
  If you add an adjective, enter the reading in its dictionary form. The ending must be 〜い (~i). For example, あたらしい (atarashii) or たかい (takai).
  If you add an adverb, don’t include inflective endings, such as 〜な (-na) or 〜だ (-da).
  
  Click the With column, then type the replacement word (up to 64 characters).
  


See the question mentioned as duplicate for methods for older versions of OS X.