You can't export starred items from Reader. The most obvious thing to do is bookmark each in your browser. I figured out a workaround, though.
In the old account (with all the starred items) make each starred item a shared item as well (Share link at the bottom of each item).
Log in to your new Reader account. Add the email address for your old account as a followed user. Suddenly your shared items will be visible in your new account! You can proceed to star them for your new account.
This may take a while, depending on how many items you have. I had about 100 items, so I had to share each one in the old account, then star each one in the new.