Addons games to the family library

[Februus]

I need your help if the game is available in the family library, can I buy an add-on for it on my account without having the main game on it?

[Spawn of Totoro]

You need to own the game in order to purchase DLC for a game. Borrowing is not owning the game.

You can, however, gift the DLC to the user who you are borrowing from and would then have access to it as well.