Utilizing multiple modules/market place items in a single game.

I'm running a few games with market place modules, LMOP, CoS and have been looking to add some sidequests via marketplace purchases, but before i do so i want to ensure i can add them to these existing games. Currently, i dont know or see a way to say add LMOP to CoS or vice versa. I do see i can add some stand alone quests from the essentials kit/icespire though via the Add Ons section.  i inquired on the fb group and people there noted it's 100% possible but they are all pro members, is this a pro member feature, and if so what's it called ? I was running through the list of difference between plus and pro but didnt see it called out or perhaps i dont understand the syntax.  TIA... -  J

Neither LMoP or CoS show up as having add-ons when you go to the add-on section of your games? That seems odd as all add-ons I believe should show up for every game provided you are the creator and have the products. Edit: did not know the nature of modules apart from things like MToF adventures, my mistake!

LMoP and CoS are not add-ons; they are modules.  Unfortunately you can't just add two modules together when starting a game.  You would have to choose one of the two as a the 'base' module for the game, then move the content from the other one into it by using the character vault, which is not an easy process for a lot of items.  If you are a Pro subscriber you can easily use the Transmogrifier tool.   Add-on items that you have access to can be added to any base module.

Thanks for the replies. So in short, i need to look for content in the marketplace that are noted as add-ons? 

Add-ons are easy  to add to any current game, however it is possible to add any content you own to any current game. A Pro subscription just makes the process easy to transfer between games using the Transmogrifier tool.