[fetchmail]altering header with fetchmail

Hi,

I am looking to retrieve mail from multiple mailboxes to one local machine
to one local use account, but for each a seperate maildir.

I would like to be able to customize the header for each messages of the
mailboxes I am retrieving from so I can use procmail to sort out which
mail should go where.

Is this possible with fetchmail? Or is there any other way that I am
overlooking to achieve the same objective?

Best regards,

Arjen