Is there a way I can create an expandible/collapsible tree in word?

Sorta. There are techniques here to mimic it:

http://gregmaxey.mvps.org/Toggle_Data_Display.htm

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Daiya Mitchell, MVP Mac/Word
Word FAQ: http://www.word.mvps.org/
MacWord Tips: http://www.word.mvps.org/MacWordNew/
What's an MVP? A volunteer! Read the FAQ: http://mvp.support.microsoft.com/