According to the documentation you do have to enable maintenance mode, I do not see a workaround I'm afraid.
Configuring CA signed certificates for ESXi 6.0 hosts (2113926) | VMware KB
Installing and configuring the certificate on the ESXi host
After the certificate is created, complete the installation and configuration of the certificate on the ESXi 6.0 host:
- Log in to vCenter Server.
- Put the host into Maintenance Mode.
- Navigate to the console of the server to enable SSH on the ESXi 6.0 host.
- Press F2 to log in to the Direct Console User Interface (DCUI).
- Click Troubleshooting options > Enable SSH.
- Log in to the host and then navigate to /etc/vmware/ssl.
- Copy the files to a backup location, such as a VMFS volume.
- Log in to the host with WinSCP and navigate to the /etc/vmware/ssl directory.
- Delete the existing rui.crt and rui.key from the directory.
- Copy the newly created rui.crt and rui.key to the directory using Text Mode or ASCII mode to avoid the issue of special characters ( ^M) appearing in the certificate file.
- Type vi rui.crt to validate that there are no extra characters.
Note: There should not be any erroneous ^M characters at the end of each line.
- Switch back to the DCUI of the host and select Troubleshooting Options > Restart Management Agents.
- When prompted press F11 to restart the agents. Wait until they are restarted.
- Press ESC several times until you logout of the DCUI.
- Exit the host from Maintenance Mode.