Here's what I would do: In site settings, make sure the avatar system is set to User Upload. Upload a test avatar and go through the code and find where the avatar images are being placed and how they are associated with the user in the database.

Once you have all of that information, depending on your needs you can either do some SQL mass updates to load and associate the images, or create a User Registered Event Handler to do it as each user registers in your system. For the latter method, I'm assuming you're using LDAP/Active Directory authentication.

Hope that helps,

Jamie