I rename my books purchased from the Sony eBook store to the actual name of the book with an 'lrf' extension. So, for example, the book
"The War of the Worlds" C985985nchgq8.lrx, would become
"TheWarOfTheWorlds.lrf" - or if you're real anal "The War of the Worlds.lrf"
I then add the book to Calibre using the "add single book" option.
At this point I can edit the meta-information in Calibre, and when I have what I want I can send the book to the PRS-505. As long as the "secret code" embedded in the book matches the device, it does not seem to care the extension is now and 'lrf' instead of an 'lrx'. Plus with the 'lrf' extension, Calibre won't complain about sending it off because it is now a known file type.
You still can not view the book through Calibre due to the encryption, but I don't view books through Calibre anyway.