Can I add subdomains to my primary domain?

Yes it can be done by an application called Multi-Country Store

This app allows you to open a multi-country online store with a dedicated domain or sub-domain for every country, and you can set product prices, special price and discounts for every country separately.

The following features are available in the app:

• When you enable the app you can add multiple countries to your store.
• Specify a default currency and a dedicated domain or sub-domain for every country.
• In product edit page, you can specify prices, special prices, and discounts for every country separately and in the currency selected for that country.

 You can get the application for free on the Enterprise plan ... Price applies to other packages at  299$

On the next link, you can learn more about the prices of plans with the applications attached to each one 

https://www.expandcart.com/en/online-stores-pricing/