Is Combining Modules into one Game possible?

I'm about to run a homebrew campaign, but it would be helpful to have the assets from the various modules I currently own on Roll 20 all together into one. Is there a way to create a game that just has all of your module assets together for us?

The only way I know of is to create a game for your campaign (or use one of the modules that you want to use), then a separate game for each module whose assets you want to use. You can then use Transmogrifier (available only with a Pro account) to copy assets from those games to the game that you are using for your campaign. The number of games that you create does not count against your storage limit, and neither does the material included with modules..

Besides what Rick A said,  Another way is if / when the Modules you own are offered in "Add-On" format in roll20.  If it's only in "Module" format then you can only Create a game with it in the first place (or transmogfrify like Rick said).  If the Module product has the "ADD-ON" functionality of Roll20 then you can add it to your existing game that may already contain other modules. Go to your Game Details webpage. Look below your game's Description, above your "Game Discussion" forum.  Look for "GAME ADD-ONS" drop-down menu, and see if you have any of the products listed in there.

Just to add a caveat. One of the reasons that Roll20 offers both Modules and Addons is to protect you from this sort of idea. Having everything in one game sounds useful, but that game would be so huge and unwieldy as to be practically unusable. It would be slow, might not even load, would certainly be uncopyable. It would also likely have issues with duplications. Go with the suggestions above and make a resource game for each module, then pull what you need from the transmogrifier.

One other thing, it is more of a mental shift:  Don't think of a game as "THE game", think of it as "A chapter", you can move characters around from chapter to chapter as necessary.   Either via Transmogrifier (Owner needs Pro) as already mentioned or via the Character Vault (Owner or Player needs Plus), or even manually by duplicating information in side by side windows (Free).