Thanks for your inquiry,
Only the Commander-in-Chief and the Staff Officer have access to convert or capture league cities. Please tap on the League icon at the right side of the battlefield, then you can see who is the CINC and there is also a small star beside the name of Chief Commander. The Chief Commander can promote other members, for example, the Staff Officer on the league member page in a battlefield. There will be a small triangle icon located beside your name after you are promoted as Staff Officer.