Form Based Authentication and Current User Filter on External List

Hello,

we have an external List (System:SQL Server). The Authentication for the extranet zone is form based on LDAP Provider.

Now, if login the user  with  Windows Authentication (NTLM) and read the external list works the Current User Filter perfectly.
Example User i:0#.w|domain\username

If the user login with form based Authentication doesn`t work the filter by Current User.
Example User i:0#.f|admembers|username

is it possible that convert the Username in first example if logged as form based useror exist a other solution for my problem?

thanks
christian