Welcome to community,
And since you have only one ESXi host and your vCSA is running on this host, you will be unable to patch this host with Update Manager for reason that you already knows: the host must be maintenance mode.
About add another host, with Essentials Kit without vMotion feature, you can try use a shared storage datastore between the two host, shutdown the vCSA on first host and power on the vCSA on the second host... and finally patch the first host.