The only way to add (whatever) words in iReal book is in Text, but you would have to copy/paste each individual line of words (Bob is right, no easy way...) presumably starting at the first bar of each system and use spacebars between each word to line them up with the bars/chords across that system.
(But remember please do not submit lyric content with any iReal book files to our forum because of copyright.)
However you could play around with creating PDF files from Word. Make a custom paper size proportional to the ipad dimensions, lay out your information to near the edges (perhaps), composite all the PDF pages into a single PDF (if not already) and email to yourself on your iPad. Open it in your email and save to iBooks.
It is possible to use Acrobat Pro (or similar) to create a titled bookmark for each song so iBooks can show you a table of contents of all the songs, where you tap on the song name and it appears.
Bookmarks