You can convert any city of yours to league city as long as you have access. But all resources of that city will be donated to your league and its buildings including army base, turrets will all disappear no matter which level you have upgraded them to. That city will also become a lvl1 league city. So we suggest you to transfer an underdeveloped city into league city.
Besides, please transfer ownership of its buildings to your other city before you convert it.