Wait... Is it possible to back up the DLC obtained via in-game store? Mostly to install in on a PC with no web access.
You cannot back up DLC content and just copy it to another computer without internet access.
However, you can install the game and DLC, start the game once after all DLC has been installed, and _then_ disconnect that computer. The DLC will then have been verified for that computer and you can play the game in offline mode.