You are searching in the wrong direction. NO USER WILL EVER ENTER HIS FACEBOOK CREDENTIALS ON ANY SITE OTHER THAN FACEBOOK.
secondly, when facebook team finds that your site is asking for facebook credentials ( the FB team reviews your website before approving your app), then it will block your application id.
There is no way to pass the facebook credentials 'internally' to the facebook API.
The only way is that facebook allows to third party sites to provide access to users data is by making the user login into the facebook ( with SSL) and then gives a unique access token which can then be used with every request to facebook.
See more here
http://facebooksdk.blogspot.com/