Best Practices for Uploading New Account IDs to Create a Mixpanel Cohort

I have a list of account IDs and I want to upload them to create a cohort for measuring metrics on these users. Typically, when I upload users with additional attributes, I first download the existing users from Mixpanel, perform a VLOOKUP in Excel to match the Mixpanel 'distinct ID' with my user list, and then upload the updated list to Mixpanel via a CSV file. However, in this case, the account IDs I have are not yet existing users in Mixpanel. These are targeted users for our product who may or may not become active in the future. How can I upload these account IDs to create a cohort without causing issues? If I upload them as they are, they'll appear as account IDs with no attributes. When these accounts actually convert and are tracked in Mixpanel with their attributes, they could end up being duplicated (one from my upload and one when the real user data comes in). what is the best way of tackling my problem ?