@Murray What are you trying to do, perhaps there’s a better way to accomplish your objective? Also, which ecommerce platform are you on?  And by “tag” I assume you mean to add a Custom Property to that list of 2000 emails.

If you really just want to tag a specific list of 2000 emails, and you have the emails in a spreadsheet or CSV file, the easiest way is to upload them into an existing (or new List) and create a new column with a property name and a value for everyone in that list. You can use a Spreadsheet or Excel and quickly drag that value down the list to populate that value to your list of 2000 email addresses.

For example, if you wanted to create a new property name called “Group” and the value is “1” you would have a CSV with a minimum two columns like this:

Email, Group
john@test.com, 1
suzy@test.com, 1
bob@test.com, 1
...

When you upload this list into Klaviyo, you’ll see that all the profiles will have a Custom Property called “Group” with a value of “1” like this:

Now, you can create a Segment of those folks where the custom property “Group” has a value of “1”.  

Alternatively, if you happen to be on Shopify, you can also add a “Tag” to the 2000 people in Shopify (they have a bulk tag tool as well).  You can see this article: Edit Customer’s Profile in Bulk.

Shopify Tags get synced to Klaviyo under the Custom Property called “Shopify Tags.”  Once the tags are in Shopify you can create a Segment like this:

@Murray another approach you could take without the export import functionality is to create a new Flow that auto adds the tag (profile properties) to those people. 

https://www.klaviyo.com/blog/start-adding-rich-data-contact-profiles-flows