Hi Pete,
there is only one way to achieve what you are looking for:
You have to write a routine (BATCH, Perl, ....) to extract the info from the text or XLS file and create an xml file in the proper format that can then be used as input for GCEventAddCmd.bat.
This single file has to contain all CIs that should be acted upon and a complete Service structure.
GCEventAddCmd.bat itself cannot perform this action, it is a CLI to send commands to SOI. So it is your part to provide the proper info first.
You also have to make sure the naming conventions fit so that the existing CIs get properly correlated with the CIs you generate via GCEventAddCmd.bat (e.g. the Universal Connector).
But you can also use a different approach to set all these CIs into maintenance (you do not need to create a Service for that):
Use WebServices calls - again, you have to provide the looping mechanism for your list.
See: Work with Maintenance Settings Using CA SOI REST Web Services - CA Service Operations Insight - 3.3 - CA Wiki
Regards,
Michael