This is a doozy. Can't believe they didn't put in a way to add one ID doc to another.
Because this was interesting I played around. All I could figure was to open an ID doc, Save As something else and trash any non text layers. I did that with 2 small docs. I made an ID Book with the 2 docs (total 8 pgs). Then I made a pdf and opened it in Acrobat. Then I saved as RTF and Word.
I got all the text but lost all the page breaks.
If you want to do this, you can then set up a new ID doc, set up placeholders for the text, apply text styles to the text, save as a Master, apply the master to everything and import the Word via Place.
Thats all sort of rediculous, but it's the only thing I can figure out.