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:
HOW TO PUT READ ALOUD INTO A FIXED LAYOUT BOOK:
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.
Liz Castro is the girl to talk to about read aloud so check her out first.
The only way you can add read aloud to an ibook is by using easy epub code editor. (Go to itunes and look it up). This app works on the ipad and is the ONLY app that can generate smil/xml code for read aloud. The only thing is that you cannot use it on anything but a raw epub file. You cannot use it with ibooks author or with any other file. Hope this helps.