I want to email a segment of existing customers who made a purchase up until a certain date, however it appears that you can only email customers who have subscribed to receive emails.
As a store owner you do have the ability to manually opt in customers to email marketing. When viewing each customer account you will see a section on the right hand side for Marketing Status.
You can change this setting individually in each customer account, bulk export to CSV and change the appropriate field and reupload (replacing existing customers with whatever is in the CSV file), or use the Bulk Editor from within the admin.
Manually opting in customers that have not requested marketing or promotional emails of any kind in order to send them marketing/promotional content that is not directly related to an existing business relationship could go against the CAN-SPAM act. You can read more about this on the Federal Trade Commission website.
Also, it is important to note that if the majority of customers choose to unsubscribe once you have emailed them or mark the emails as spam this will drastically lower your trust within the email service Shopify offers and could result in a restriction of Shopify Email for your merchant account.
I just want to make sure you have all the required information before you move forward with this decision.
Let me know if you have any additional questions about making this change to your customer accounts or with using Shopify Email.