Go to 'Settings' of your Gmail, then click 'Accounts and Import' tab. Scroll down a bit, then click 'Add a POP3 mail account you own' link. On the pop-up page, key in the email id that you want to import, then click 'Next' button. On the next page, key in password, POP server and port, select 'Archiving incoming messages(Skip the inbox)', and you also can change the label for the incoming message. Now, click 'Add Account' button, and an activation email will be sent to the email that you want Gmail to delegate. Follow the instruction in that activation email, you'll successfully grant Gmail to receive and send message on behalf of that email.

Pretty simple, right? Repeat above steps to add all your email accounts to Gmail, and enjoy a 'login once, see all' experience on your email management!