Add another custom post type into the main blog post archive loop. #WordPress

add-cpts-to-post-archive.php

/**
 * Add a CPT into main post/blog archive
 */
add_filter( 'pre_get_posts', 'chuckreynolds_add_cpts_to_post_archive' );
function chuckreynolds_add_cpts_to_post_archive( $query ) {

	if ( is_home() && $query->is_main_query() && empty( $query->query_vars['suppress_filters'] ) ) {

		$query->set( 'post_type', array(
			'post',    // assuming you still want normal posts in there
			'cpt-name' // registered custom post type name
		));

	return $query;

	}

}