You can write a macro to save a file as GB2312 (my sample as below), and then use the Advanced Open on the File menu or the Run with Temporary Options on the Macros menu to specify all the files to convert.
document.Encoding = 936; // Chinese Simplified (gb2312)
s = document.FullName;
nExt = s.lastIndexOf( "." );
nBackslash = s.lastIndexOf( "\\" );
if( nExt == -1 || nExt < nBackslash ) {
nExt = s.length;
}
s = s.substr( 0, nExt ) + "-gb2312" + s.substr( nExt );
document.Save( s );