Accessing network when using windows authentication in web app

[Pete]

I am using windows authentication in Apache to access the web page -
work Ok but I need
to use this authentication, username and password to access an external
network to get some data.
How do I extract the username and password from the windows
authentication ( for a user) and then use this access the external
network.

I prefer not to use impersonation as the username and password are in
the web.config. Would prefer to use the users username and password.

All help is greatly appreciated