add_action( 'pmxi_saved_post', 'soflyy_add_password', 10, 3 );

function soflyy_add_password( $id, $xml, $u ) {
	if ( $pass = get_post_meta( $id, '_my_post_password', true ) ) {
		$post = get_post( $id );
		$post->post_password = $pass;
		wp_update_post( $post );
	}
        delete_post_meta( $id, '_my_post_password' );
}