Send the MERGE field enddate to MailChimp.

pmpro-mailchimp-merge-fields-enddate.php

function my_pmpro_mailchimp_listsubscribe_fields($fields, $user)
{
	$level = pmpro_getMembershipLevelForUser($user->ID);
	
	$enddate = date('Y-m-d', $level->enddate);
	
	$new_fields =  array("ENDDATE" => $enddate);

	$fields = array_merge($fields, $new_fields);
	  
	return $fields;
}
add_action('pmpro_mailchimp_listsubscribe_fields', 'my_pmpro_mailchimp_listsubscribe_fields', 10, 2);

/*
	(Optional) Tell PMPro MailChimp to always synchronize user profile updates. By default it only synchronizes if the user's email has changed.
	
	Requires PMPro Mailchimp v2.0.3 or higher.
*/
add_filter('pmpromc_profile_update', '__return_true');