As I understand it, some products are so large they cannot be added to an existing game. This is the main difference between a module and an add-on. Yes, with a Pro subscription the Transmogrifier is handy to import part of a module into a game. You can use the Character Vault with most (any?) adventures purchased on Roll20, regardless of your subscription level. You can use that to transfer your player characters, and any special NPCs or monsters. Moving maps, handouts, playlists, macros, etc., will require the Transmogrifier and/or copy and paste.