This cannot be done by InDesign or any Adobe application I know of.
There are some third party software which claim to be able to do it. Do a Google search on "pdf to epub".
That said, unless you just have straight text in one text flow, such software will probably produce a very poor, substandard EPUB file. There is not enough structure in most PDF files, and EPUB is akin to HTML: It needs structured information.
As much as I hate to leave InDesign, if all you have is a PDF, you need to go outside the program.
Do a search on the web for "PDF to ePub" and you'll find a lot of choices.
You're right ahead of me. (But I had to stop to take a phone call.)
Great minds think alike. ;-)
I don't use ID for ePubs, but have been in the same situation. I used acrobat to e port out a Word document and imported that, massaged that, did some CSS work and the epub came out well. I suspect I would try that workflow from acrobat to ID if I had to.
I assume that you are interested in *reflowable* epub, but might I ask what is your interest in EPUB3 fixed layout?
If you could place all those PDF pages in InDesign and export as a fixed layout viewable on a variety of tablets...would that meet your customer need?
It's nice to see you're interested in this.
A different thread popped up the other day (http://forums.adobe.com/message/5565092#5565092) where the person posting wanted something that would be EXACTLY a fixed layout ePub and wanted to know if ID would be right.
Notice down the thread where he is worried that his panels might change position.