Yes, you can use your own or any third party cron service. Disable the cron in Acymailing->Configuration, otherwise you'll have two cron servrs triggering your Acymailing and that will lead to issues.
You can use the cron url on any cron server. It will trigger your Acymailing and Acymailing will take care of the rest once triggered.
Please note that you need the Enterprise version and a valid subscription in order to be able to use the cron service, regardsless if it's running on Acyba's or any other cron server.