After a bit of looking I found that there really isnt a way to transfer maps in roll20. Being able to transfer a map you already have made to another campaign would be a great help.
You can transfer maps with the transmogrifier, but you need the Pro subscription level. That being said, having a common repository of maps for all your campaigns is not a bad idea. I find it pretty tedious to check old campaigns to see which has what in order to transfer.
You can make a game for the specific purpose of being your repository.