Use the following filter hook to set your own custom icons URL to override the default Freelancer Icons and add your own.

freelancer-set-custom-icons-url

/*
 * Edit the add_filter code below to add
 * your own custom icon set in place of Freelancer's.
 *
 * NOTE: Do not define the FREELANCER_ICONS_DISABLE
 * constant if you want to use the filter code below.
 */
add_filter( 'freelancer_icons_url', function() { return get_stylesheet_directory_uri() . '/icons/css/custom-icons.min.css'; } );