No need to convert them. They work eather like p3t or p3t.edat extension. When decrypted, they get to be edats actualy but nothing is changed as far as functionality or install procedure is concerned.
I forgot if there is an simple way to install them on OFW. One of the methods is data transfer but this requires two ps3. One with OFW and other on CFW. If you can install themes via USB on OFW then try puting themes on USB in "THEME" folder wich you create by yourself. "THEME" folder must be in caps letters.
Then go to Themes submenu in XMB and on right top you will see "Install" option. Press and see if OFW will recognize them on USB.