If I were you I'd be tempted to leave all my old email behind: just because it can be saved is it really necessary to hang on to all that stuff?
All else aside, I thought Blake already hit on the solution....
.... Yahoo has an option to interrogate third-party mailservers and then to download the mail from them to the Yahoo server.
You need to:
a) Have a Yahoo account (duh
b) Log in to Yahoo mail and click the "options" button on the left
c) On the resultant screen click "Check Other (POP) Mail"
d) On the next screen click the "Add mail server" button.
e) Beforehand you need to have checked that you know the university mailserver name(s) and your password there.
f) Add the university mailserver name where it says "Mail Account Server"
g) Add your username where it says "Mail Account Username"
h) Add the password where it says "Mail Account Password"
i) Uncheck "use filters" and "retrieve new messages only"
j) Click "OK" at the bottom.
k) From the page which shows your stored details click the "check mail" link on the far right. That should download everything that is stored on the university server to Yahoo.
Two or three additional points. I don't know anything AT ALL about your university server so if you have problems with server names, passwords etc etc you'll need to ask a systems administrator there.
I am also ASSuming (1) that because you have an IMAP system that all your emails are still stored on the server there and (2) that they are downloadable using the Yahoo system.
Let us know if/how it works out!