Hi,
Only addition is possible, you don't want random people deleting your customers
Here's an example on how to send a new customer to mailwizz via api https://github.com/twisted1919/mailwizz-php-sdk/blob/master/examples/customers.php.
Please note that when a customer is added via the API it is subject to the same rules as if he is subscribed from the web interface.
Thanks.