The only way to play a game which has 4+GB files is to transfer it to the internal HDD - you can't play it off the external.
So first make some free space on the internal. Use a window application like split4G or comgenie's split to transfer your game to the external FAT32 formatted HDD. These 2 applications will split the big files to comply with FAT32 4GB restrictions.
Then attach the external USB HDD to your PS3, launch comgenie's filemanager and transfer the game to the internal hdd. During the copy process all split files will be joined back automatically by the filemanager.