Ash here from the Shopify team. Thank you for your post in the Shopify Community!
I'm happy to help out with adding a subdomain to your existing account. We have some awesome documentation in our Help Center regarding domain set up, and a section specifically for connecting a subdomain. That page has step-by-step instructions on setting up a subdomain for your shop. If that doesn't quite answer your question, then in order to provide the most accurate assistance I need to gather a bit more information from you first. If that's the case, could you please reply back to this post and let me know the following:
I look forward to hearing back so we can get your subdomain connected.