How do I use 3 marketplace "campaigns" for one larger overarching "campaign?"

(TLDR: please scroll to last paragraph to skip my introductory description and see what I am asking for assistance with.) I am working on a 5e game with a Fae overarching story based around the influence of Neverland on multiple 'worlds,' Greyhawk in the area of Ghosts of Saltmarsh, Forgotten Realms in the area of the Moonshae Isles, for which I am still trying to decide which published "campaign" level resource to use, mid Renaissance London as more steampunk than either Greyhawk or Forgotten Realms. The first crossings into the Fae realm and are using The Wild Beyond the Witchlight and crossings will increase in difficulty with Neverland being the primary source of Fae abductions but the players, being adults, will have to try multiple ways to reach Neverland. None of the ways to Neverland will be stable due to the influences of Peter Pan's and Queen Mab's variable relationship between friendship and strife. So, the players' characters will be travelling between multiple marketplace published "campaigns," and I am trying to figure out how to fuse them under the overarching campaign "Much Ado About Neverland," and yes, the implications one of the title's authors means that Oberon and Titania have influence and could possibly be encountered, but trying to fight either would not go well for the player characters, Peter Pan and Queen Mab are beyond the players being able to handle if they try to directly battle them and Titania and Oberon are higher powers than either Peter or Queen Mab. So, I am trying to figure out how to use my already created Much Ado About Neverland and import everything from Ghosts of Saltmarsh, The Wild Beyond the Witchlight, and whatever marketplace published Forgotten Realms setting book I decide to use, or do I have to start a create a game for each and juggle the characters between 4 different games. I am a pro subscriber but I don't think my players will be so I don't know if their characters will be easily portable between games if the game has to change every time they make a crossing to or from the Fae realm. Any ideas how to best manage this would be greatly appreciated.

I skipped to last paragraph as instructed. Yes you probably will-should use Character Vault feature of Roll20 to, as you said "juggle the characters".  Good news yes your Players will be able to do it, without being PRO themselves, for 2 different reasons.  First reason is you (the GM-Creator) has Pro, so that settles it. In addition to that, also, most of these modules from Roll20 have a feature called "All Access Vault Enabled" so that people can import Characters from Character Vault anyway. It won't be much of a "juggle" (back-and-forth) if you play each module on its own, sequentially.  Granted I understand if you wanted to make an open world that has ALL of the adventures, then yes you'll end up needing to juggle with Character Vault. However.  If-When the Module that you want is offered from Roll20 Marketplace as "An Add-On" product, then YES you can install Add-On modules onto your larger game that already has a base module! Look on each of those products (Saltmarsh, Witchlight) and check to see if they are offered in "Add On" format.  If yes, then you can do what you wanted in the first place. Some of the larger modules are NOT offered as Add-On, for the reason mainly that they're too large (vast amount of content) to combine. So I guess, Step 1 check if you can install the modules as "ADD ON".  If not then you should be fine using Character Vault.

UPDATE /  additionally, another option since you have PRO you can use Transmogrifier, that's another way to move Characters between 2 game tables. 

Thank you for your reply, you are well informed and have greatly helped, I really appreciate your taking the time and as a first time poster the curtesy and thoroughness of your reply, a great welcome on behalf of the Roll20 community.