I'm guessing that you've checked already into BookDesigner, and it doesn't support creation of protected content for eReader.
I believe that there is a commercial program for creating ebooks which could be used. Here is the page I was thinking about....
http://www.ereader.com/products/ebookstudio, but it doesn't say anything about locking or encrypting books. You might need to try the demo software.
Ooops... never mind. I see after posting that this was already covered above.