Exporting files and subfiles from Monday.com to a CSV

Description

I have a Monday.com database for CRM and Engineer management which has had one to numerous subitems created for many/most of the customers. I am trying to load our systems into a new engineering system which needs a flat CSV file as import. I can export a flat CSV file of the customers themselves from Monday.com and can also optionally include sub-items but these then appear as separate lines on the resulting CSV file without an index to the parent customer record that I can see.Is there a way of exporting the subitem files held in the Monday.com CRM to a CSV file with an index to the parent customer file so that the two can be re-united in the new database the other end of the export?(See attached sample customer file with embedded subitems....)
Can anyone advise or offer the help need to do this?
Many thanks
Simon