Use the `idx_plus_data` filter to define another variable.

use-idx_plus_data-filter.php

<?php

// We want to add a new value for %%short_excerpt%%
add_filter( 'idx_plus_data_short_excerpt', 'add_short_excerpt_data_to_idx_plus', 10, 2 );

/**
 * Modify the data output
 *
 * @param mixed $data Data to return
 * @param string|null $key Data key, if set
 */
function add_short_excerpt_data_to_idx_plus( $return, $key ) {

	// Then get the excerpt
	if( $excerpt = IDX_Plus::get_data('excerpt') ) {

		// Shorten it for the return value
		$return = wp_trim_words( $excerpt, 10);
	}

	return $return;
}