I have searched through the Civilopedia and the Tech Tree, and I can't find any mention of which technology enables a player to settle a city on a marsh tile. Because of this, I would suspect that this wasn't allowed... except that the Civilopedia entry for the mangrove modification to the marsh terrain states that the mangrove is removed from the tile if an emplaced city on the tile grows to size 6. This strongly implies to me that there is a possibility, somewhere in the game, of enabling marshland settlements.
Just as a suggestion, you might consider adding to the terrain entries in the Civilopedia which technologies or other circumstances convert unsettlable terrain to settlable.
Just as a suggestion, you might consider adding to the terrain entries in the Civilopedia which technologies or other circumstances convert unsettlable terrain to settlable.