Rime of the frostmaiden

how do i add this to an existing module/game?

Create a new game with all of the module in it, then transmog things over on demand to your new session with the transmogrifier.  

Modules cannot be added to games. They are so large that adding them to existing content runs the risk of bloat. Instead, they are designed to be the starting point of a game. Smaller adventures are provided as addons. Unless you only need a few specific things from the module that you can use the Transmogrifier to add to your existing game, your best bet is DM Eddie's advice.