IMAPSize is a tool to manage IMAP mail accounts. It is not a mail client. You can use it to locate folders which consume much space and manage then, to avoid storage quota. IMAPSize offers a variety of useful e-mail tools for this, and it can also convert e-mail between eml (Windows Mail/Live Mail) and mbox (Thunderbird) format.
You could use it to import mail between Windows Mail and Thunderbird/Evolution.