Instead of creating a lookup file from the .csv that is provided, why not just ingest the .csv file into an index? Something like this:
1) Setup a new input file monitor to watch for the provided .csv files to be added to a directory.
2) Have the other team copy any new file into that directory.
3) Splunk ingests input .csv file into new sessidcsv index
4) Create a scheduled report that correlates the two indexes
5) Provide access to this report by the teams or have the report e-mailed back to the group that provided the .csv file.
There's always more than one way to do it. But, this is more 'self-service' for your groups.