The devil is in the details, but in general I would think you have two
primary directions to look at:

1. Converting the CSV file into a format the users want to see,
possibly even abandoning EXCEL from their usage of the output.
2. Converting the CSV file into a format that EXCEL can manipulate into
various ways that the users want to see the data.

Which direction you go would primarily depend on how much interaction
the users need to have with the converted data -- how many ways they
want to "slice and dice" it.

As long as combining rows of data is complex, I don't see much of a
choice other than to "massage" it for them ahead of time.