Yes. Check out Kovid Goyal's libprs500 project supports the Sony PRS-505/500 and it's GUI frontend Calibre.
It has very good version tools allowing people to convert eBooks previously stored in HTML, PDF, TXT, Microsoft Reader (.lit), IDPF/Open eBook (.epub) into Sony's format, and with a little bit of search on Google - you may also find a converter for the Secure eReader format.