Yes, it is possible to add external email accounts in SmarterMail for the domains which are not hosted on your Windows VPS. There are just two steps to accomplish this. Please find them as below:
Step - 1:
Add mail "A" record and "MX" record under the DNS zone of your domain at your DNS provider control panel. You can get record information from this article.
Step - 2:
Once the DNS records are added, you can add your domain name in SmarterMail. You can refer to this article for step-by-step guidance.
