Something like this..
| map search="search index=_internal| fields host,sourcetype,source|outputlookup $myCSVFile$.csv"
Above is an example.. Change it to your own searches to generate the xyz field and then apply it to the output of another search to generate outputlookup
Where is your field located ? Is it in an index ? Are you trying to combine fields from multiple sources ?
Official documentation for output CSV is in Splunk docs :
So all you have to do is make sure your field is there before you run the outputcsv command:
index=yourindex sourcetype=yoursourcetype | fields fields_you_want_to_keep | outputcsv MyTestCsvFile
Pease provide some sample data if you want you help with your specific SPL.