Using IMAP with a local master copy?

[Dave]

Thanks for the thorough treatment of IMAP issues. I currently access
all my mail accounts via POP, but IMAP is appealing and I'm hoping to
switch. However, I would like to keep the master copy of my email and
folders on one of my local machines, in part because I'd prefer to have
it under my control, in part because it will get larger than the
available space on my provider, and in part because it will make
switching providers easier. I know I can keep local folders of messages
no longer on the server. But is there some way to have a folder
containing both server-side and local-only messages? For example, if my
Friends folder contains 500 messages over 3 years, can I somehow keep
the last year on the server but keep all 3 years' worth in the client
copy?

Thanks.