I have seen it several times where computers could not be encrypted with FileVault 2 as there was no Recovery HD on the machines. In many cases, end users are already actively using the machines which makes re-imaging a hassle. Due to that, I have been looking for a way to create a functioning Recovery HD without re-imaging. The Googles have not given me much to go with until I found this yesterday:
Tried it out yesterday on a 10.9.2 Macbook Pro 13" and it worked flawlessly! Keep in mind you'll need a copy of the Install OS X Mavericks.app on the machine to in order to grab the InstallESD. I was able to encrypt via FileVault 2 once everything completed. Please note that I don't know the creator (@csilvertooth) nor claim to back the scripts he has made. Just wanted to share this out as it worked for me and thought it could benefit others.