I belive so. I guess it's the search for external id, or is it all of it?

This function does that search

    public static IEnumerable<IMember> GetMembersByExternalId(int externalId, bool includeLockedOutMembers = false)
    {
        var members = ApplicationContext.Current.Services.MemberService.GetMembersByPropertyValue("externalid", externalId);

        if (!includeLockedOutMembers)
            members = members.Where(m => !m.GetValue<bool>("umbracoMemberLockedOut"));

        return members;
    }

The login tries to login to the membership provider. If it fails, try to get the user by externalId, and login to membership provider. If that also failes, it will also try to fetch the user from the external db. If successful it stores the external user into umbraco, and adds the externalId to the member (custom field) and login that user. I think it's something like this... (may be some error in the code below, but it may bring you on track)

var user = GetMemberByExternalId(externalId);
if (user != null && Membership.ValidateUser(user.Username, password))
            {
                umbracoUsername = user.Username;
                return true;
            }

var externalUser = userService.GetUser(externalId);
if(externalIUser == null) {
    return false;
}

if(user == null && externalUser.password == password)   // we don't want to create if user is found but with wrong password and also if password don't match external db
{
    user = CreateMember(externalUser);   // function that creates the member in umbraco
    if(Membership.ValidateUser(user.Username, password))
    {
        return true;
    }
}

return false;