The official website for The Settlers of Catan

Question about Barbarians & Traders Dogma - Can I place a Witch on a Building that already carries a Dogma?

Answer

Yes. You definitely may first play a Dogma and then build a Citadel and use the latter to play a Witch.

Note: The Citadels and Witches belong to the theme set "Wizards & Dragons", which is part of the Catan Card Game – Expansion Set.