Just create a new mailbox in Mail on OS X, for location, select "On My Mac". Give it a name that makes sense - like old email, 2007-2008 email, whatever. Under that, create a structure of mailboxes that makes sense, like for sent and inbox, or whatever else. Then just sort your inbox/archive, wherever you have the old emails on Apple's servers, and drag them to the appropriate local folder...it will download a copy of the messages and delete from the server. That should free up some space.