spliting up old sent-mail, postponed-msg, trash + others mail folders in

Hi all.
Im new to this list. 
The subject of this message is pretty self-explanatory, aint it =)
I've been told I could to this with formail, but I check doc a little, and I 
have not found the exact way to do the thing. 

We used to have our own maildir directly in our home here at the office, we 
recently changed our system, and installed Exim. Now, our home directory is 
seperated from our mail dir. I would really like to import my old folders in my 
current maildir. I was using Pine then. 

Anyone has any clue ?

Thanks for any help