Yes, you can add read aloud to a completed iBook but you will need to add code (XML) to it. Here are some links on how to do it.
MAKING A FIXED LAYOUT BOOK:
http://authoradventures.blogspot.com/2012/01/how-to-create-fixed-layout-ibooks-p art.html
HOW TO PUT READ ALOUD INTO A FIXED LAYOUT BOOK:
http://www.elizabethcastro.com/epub/
http://www.amazon.com/Read-Aloud-iBooks-Elizabeth-Castro/dp/1611500176
She is the only person right now who actually tells you step by step how to do it. I bought her book from Barns and Noble (but you have to order it because its a print on demand book). Its great and the only book I could find on the subject.
Also, if you don't want to touch code at all, this site called Kitepress will do it for you. They even send it to itunes for you and put it on Amazon for you too and do a little promotion by putting your book under their publishing name so all books link to ones made using Kitepress. (Don't know if this is good or not but something to think about) But they take a percentage out of any of your profit when you sell it.
http://www.franklinjr.net/kitepress/kitepress-how-it-works/
Liz Castro is the girl to talk to about read aloud so check her out first.