I tried to apply mentioned workaround:

"Import your CSV file

To import your CSV file:

1. In the navigation on the left, select Customers to import individual customer details or Organizations to import organization details.

1. Select Import details.

1. Select Continue.

1. Select Choose a file and select your CSV file.

1. Select Start import."

However, step 2 is invalid since there is no Import details on the Customers screen.

Time to roll up my sleeves and do changes of customer names manually!

https://getsupport.atlassian.com/browse/JST-905045

Well, at least one of the methods Atlassian supports as per this link is not working:

When adding a bulk of customers (whether it 200 or 1000) the system randomly drops email addresses to be added.

In combination with JSDCLOUD-8982 and JSDCLOUD-9376 it's pretty much impossible to find out which customers are missing.

Another bug that I found (no ticket yet): If you have multiple customers for an email domain you can search for that domain but only some customers are found. Searching for individual email addresses of the same domain which were not found before, does then show the customer,

Overall, this is very unreliable and we are now waiting for Atlassian Support to add customers for us (who btw also use the UI so we are now going through the customers and tell them which ones are missing as the bulk import dropped them... wow!)

It really is unbelievable...

Its 2021, bulk upload from a csv template is basically every application know to man. How does this not have it? Absolutely inexcusable. 

Not an API solution, but here are the methods Atlassian currently supports:
https://confluence.atlassian.com/confeval/jira-service-management-evaluator-resources/jira-service-management-bulk-add-customers

I should have explored this prior to purchasing Jira. Being able to bulk add users is a must for large organizations.

This would have been a deal breaker.

Hey Atlassian, any movement on this? This is just barely workable with my current number of internal customers, but if the company grows much, we'll need another solution.

There is a separate channel, but all requests seem to end in them opening ine of these tickets. We have yet, in the years that we have been using them, seen any of these tickets implemented. Even in the case where it has gotten hundreds of votes from other customers.

One example, look for a ticket about drag and drop for reordering of subtasks. Its literally been open for years and is one of the most voted on. They finally got around to implementing it on Jira Server, but for some reason, its been months and it still hasnt been rolled out to cloud. Its literally 10 lines of javascript code that they would have to integrate into the cloud websites.

Thanks Benjamin. Thought so, theres so much hanging requests without a single comment. I really like Jira Software but I guess im going to have to start considering alternatives for our SD.
Have you tried making direct requests to Atlassian (or better said: Do you as a costumer have a diferent channel to communicate with Atlassian)?
Cheers

I have yet to see Atlassian make any improvements that customers have asked for. We have been on the platform several years, and although its fairly powerful, we are frustrated by the lack of material improvements.