When adding a service using the Service Manager Dialog an entry is added to the .SMD file in the bin directory of where the Service Manager is running. This file can be updated manually without using the Service Manager Dialog.
Please see the link to the documentation below which defined the syntax that can be used to update this file:
https://docs.automic.com/documentation/webhelp/english/AA/12.3/DOCU/12.3/Automic%20Automation%20Guides/help.htm#AWA/Admin/admin_SettingUpSMDFile.htm?