<?php
# add this code in your functions.php file of your theme or child theme
add_filter( 'wcfm_product_manage_fields_general', function( $general_fields, $product_id, $product_type ) {
    if( isset( $general_fields['is_virtual'] ) ) {
        $general_fields['is_virtual']['dfvalue'] = 'enable';
    }
    if( isset( $general_fields['is_downloadable'] ) ) {
        $general_fields['is_downloadable']['dfvalue'] = 'enable';
    }
    return $general_fields;
}, 50, 3 );