A Hero type that wouldn't normally be able to learn a certain Secondary Skill can still do so at a Witch Hut. For example, a Necropolis hero normally cannot learn Leadership. A mapmaker may allow Leadership to be learned at a Witch Hut which could allow a Necropolis hero to attain this skill. Similarly a hero who would not normally be able to learn Necromancy can attain this skill at a Witch Hut.
Since you can't turn down the skill being taught at a Witch Hut, it might be a sound strategy to send forth a scout to investigate before sending your main hero over.