I can't find anything in the rulebook. My guess is that you would add the transposed and 5 random legendary cards into the treasure deck after the first mini boss as usual. Then that is your treasure deck for the rest of the campaign.
Shuffle all transposed and legendary together to make a deck. After the first mini boss draw five cards per player (character) from the transposed/legendary deck and shuffle them into the current treasure deck. Repeat after the second mini boss.