You can't transfer maps and costumes from one module to another. You can't save costumes to your hard disc.
Your best bet is to take first quest of your campaign, duplicate it and rename to something like "templates" or "resources" or whatever, then delete everything inside, except for things needed in other quests of your campaig, like costumes and maybe certain maps.
Use this basic "template" quest for duplicating and making each subsequent quest from it.
Comments
The Foundry isn't very user friendly and it's really still pretty annoying at this point
Your best bet is to take first quest of your campaign, duplicate it and rename to something like "templates" or "resources" or whatever, then delete everything inside, except for things needed in other quests of your campaig, like costumes and maybe certain maps.
Use this basic "template" quest for duplicating and making each subsequent quest from it.