You can install it to the usb drive and transfer it over in the data management options, overwriting the original DLC.
But keep in mind that if you do, it erases your current save for that game. Be sure to back it up if you can before you replace your legit dlc.
I believe it'll need to update again, but once it does the dlc should be intact and ready to go, assuming you have iosuhax running.
... Just in case you haven't received your answer yet on the other thread you hypothetically may have made.