Q: The Captain’s Hat and the Treasure Map cannot be Plundered, but can they be stolen using Steal A Keeper?

A: Yes. “Plunder” has a very specific meaning in Pirate Fluxx, namely the power granted by the Plunder Rule. Steal a Keeper (and similar effects, like Exchange a Keeper) are not Plundering, they are stealing. So you CAN use the Steal a Keeper card to take away the Captain’s Hat as usual, but you can NOT take it with the free Plunder allowed by the Plunder rule.