How to move archived Outlook for Windows Files to Apple Mail

While I don't know if AppleScript can do what you want, there is an Apple Mail plug-in called Mail Hub that will do what you want as well as add additional functionality. I'm a satisfied customer.

Mail will be stored in ~/Library/Mail. Gmail and Yahoo typically use IMAP, which means the messages are held on their server until you delete them. Doubtful that this is what is eating up a big chunk of your drive space; my Mail folder weighs in at just under 9GB for nine accounts, one of which has messages going back 8+ years. There are a few options in Mail/Preferences/Accounts on how to handle mail storage, but nothing that will limit the time period for what mail messages are stored locally. Removing the accounts from Mail then expunging any of their associated files will clear up some space, but I'd be looking at other places for what is using disk space. Movies and music are big space hogs, for example.