Is it possible to run sieve against an existing mailbox?

My application: I've got a mailbox that stores status messages from multiple sites. It's easier to check up on recent events by having them all in one folder. However, after I've read them, I'd like to have them archived to individual folders per site. I currently have a sieve script that uses "fileinto" to sort the mails - but I'm thinking of removing it from deliver if I can run it as a manual job later.

I suppose another method would be to duplicate the mails on delivery into both the main folder and the per site folders, then delete after I've read them - but for whatever reason that strikes me as inelegant.

Daniel