I am not sure if I understand your question, but I certainly
would prefer to get the original header back with as few changes
as possible except additions of entirely new lines of fields.
A new "received" line on top to show that it came via a certain
LISTSERV list is certainly very useful. If you feel the need
to add new fields, and these new fields can be represented
according to RFC822, then it is best to use the RFC822 format.
Otherwise, some format like X-....: is best for added new fields.