Which Yahoo are you using, eg @yahoo.com, @yahoo.ca, @yahoo.co.uk, @yahoo.co.au?
Is it a Free or Paid account? I don't think that any of the Free Yahoos offer imap. You can get forwarding or pop with some of the others. Or you have the option of fowarding incoming mail. I haven't heard that Free yahoo.com has either pop or forwarding.
I used to see posts about programs that would pop yahoo.com mailboxes, but these would have to be installed on your computer. But, from the nature of your question I guess that you do not have your own computer.
GMail cannot pop other accounts at this time.
There is a thread about automatically popping an account and forwarding the messages ...
http://www.emaildiscussions.com/...threadid=37910
But you just want a one-time mass forwarding. So
maybe someone else has a suggestion.
Before you pop your Yahoo mailbox, make sure that the method you use leaves the mail on the server. Just in case you have to try again.