I was wondering the same thing and so I decided to research it a little and I tried a few things out. I don't have many ePub books, but I've purchased a few from Barnes & Noble, and install the windows mobile B&N eReader application to my HTC Touch Diamond2 a few weeks ago.
I've read that the BN.db3 file is a SQL lite database. If you know anything about SQL Lite (I don't), you can modify the file to add new books to the database and then you can open your books.
Or you could download some free ePUB books from B&N's website. Then rename your books to whatever those free books are called - essentially replacing the book you downloaded with your book. The title would be all wrong, but when you select the book in the B&N eReader software, your book would open up.
It might just be easier to install
Freda to your phone.