I have done this a couple of times. Why did setting the password directly did not work? Did you use the ChangePassword microflow in Administration/User Management/Admin?
And did you use the latest Excel importer, because now commit and delete events wil be triggered.
[EDIT] It is good practise to just import all the data in a shadow entity first and from there create or update the user account. The new password policy could also the fault why it is not working because these errors are not shown with a direct import but will be seen when processing the data after it has been imported in the shadow entity. So do the import in a shadow entity, look up the user and create the new user or update an existing user. Do not forget to make the new user active and setting the right user roles. But this way I was able to set the Password Hashstring to the password that was imported. Hope this edit helps in finding the cause of your problem. Or find me on linkedin and I can give you a test project as example.