In order to create a system user using runmode (sling:OsgiConfig) with acs-commons, create a config as below.
1. Create a sling:OsgiConfig under /apps/project-name/config/com.adobe.acs.commons.users.impl.EnsureServiceUser-customServiceUser.xml
2. Add the below code in config file.
Once you deploy the code , you can see the system user created in your useradmin with the name you specified in the config file.
Note: You can make modifications as needed.