add_availability_date_when_backorder.php

<?php

function add_availability_date_when_backorder( $attributes, $feed_id, $product_id ) {
$wc_product = wc_get_product( $product_id ); // Get the WC_Product Object
	
	if ($wc_product->is_on_backorder() ) {
    		// add availability_date attribute
    		$attributes['availability_date'] = gmdate( 'c', strtotime( '+7 days' ) );
    	}
	
	// IMPORTANT! Always return the $attributes
	return $attributes;
}

add_filter( 'wppfm_feed_item_value', 'add_availability_date_when_backorder', 10, 3 );