Bitly does not currently offer a way to merge Bitlinks into existing accounts. If you shortened a link while logged out of your account we are unable to merge that bitlink into another Bitly account.
However, we have a trick to help track these stats! You can find stats on your Bitlink on the Bitly info page. You can see this page by adding a + sign to the end of a Bitlink. For example: https://bitly.com/QUXWD6+