So, some scenarios have special loot cards that you get at the end if you win. You can add these cool cards to your deck when you earn them.
But what if you don't, or perhaps even CAN'T, add the loot card to someone's deck right then? Can you add that loot later as if it were Basic--you did earn it after all--or would you need to go through that scenario again to get it back?
(Also, Mike et al, our group is having a blast with PACG. Thanks for making such a fun game! :D )
You definitely don't get to treat it as basic. Here is how I see it:
If you win the scenario, you've earned the loot. Whether you keep it is up to you. If the loot is an item and you don't want to get rid of the items you already have, then you've chosen to not keep the loot. If your character(s) has a deck list that allow for 0 items, you've still earned the reward, you just couldn't keep it. Either way, the loot goes back in the box with all the other loot.
You may not gain the reward from a given scenario, adventure, or Adventure Path more than once unless the reward specifically tells you otherwise.
So, you've earned it already, you can't earn it again. What you can do is apply the typical rule that if you are short cards after starting deck 3, you can choose cards of that type from the box that are two adventure decks lower than your current adventure. This applies to loot, as confirmed by Vic over here.
So, that is really the only way to get loot that you pass up. Wait two adventures, then choose it if you are short when rebuidling your deck.
So, think long and hard before deciding not to keep loot. And good luck on your adventure.