Add a custom word to Google Japanese keyboard

I am wanting to type a word in Japanese that has a unique reading (not the standard onyomi or kunyomi). As such, I'd like to add it to the Google Japanese keyboard so that I can simply type the kana, and the desired kanji will show up. At the moment I have to type out each individual kanji by their standard readings to write the desired word. How can I add a custom reading to the Google Japanese keyboard? For clarification, please see Tip #9 on this page for an example of how to do this in Windows: http://nihonshock.com/2010/04/12-japanese-ime-tips/