You can use MultiMAN to convert them directly on your HDD. Find your JB folder game in the games menu of MultiMAN, press triangle on the game to open up the side menu and you will see "Convert to ISO" at the bottom of this menu. When its converted make sure its in the folder PS3ISO on the interanl HDD and your done.
You can also use any of the other Backup Managers to do this as well.
The longer way to do it is to transfer the game to your PC, either via FTP or copied to an external HDD, and use PS3 ISO Tools to do this on your PC then once done either FTP or transfer to HDD, and put them in the PS3ISO folder on your PS3's internal HDD.