It's quite possible. I've done it several times, but it's not for the novice.
Disclaimer: If you brick your device, I'm not responsible.
Here's what is required:
1. USB connection cable.
2. iExplorer app (
http://www.macroplant.com/iexplorer)
Process:
1. Install iExplorer
2. Connect your device.
3. Using iExplorer, navigate on your device to /var/mobile/Applications/(Nook app*)/Documents/
4. Copy the *.epub file here.
5. Your *.epub should now be listed when you open the Nook app.
*All of the applications are given obscure folder names. You will have to click into each one until you find the Nook app.