There is no "smart" way. One has to read the existing file and write the new. If the file isn't too large this could be a starting point.
fid = fopen( old_filespec );
cac = textscan( fid, '%s', 'Delimiter','\n', 'CollectOutput',true );
fclose( fid )
fid = fopen( new_filespec, 'w' );
for jj = 1 : insert_here
fprintf( fid, '%s\n', cac{jj} );
end
fprintf( fid, '%s\n', new_line );
for jj = insert_here+1 : length(cac)
fprintf( fid, '%s\n', cac{jj} );
end
fclose( fid );
