Hi @unhyun.lee
Welcome to the Auth0 Community!
Reading through your use-case, iteration count is indeed not supported at this moment, also mentioned in this following Knowledge Article on Importing password hash made by PHP fails. I am wondering if it can be an option for you to have the users reset their password after importing them in Auth0. Based on what this article mentions, user import with an unsupported hashing algorithm will need to reset their password, which indicates that the import should complete successfully in this case.
This being said, have you tried using the Auth0 WordPress Plugin by any chance? While this does not revolve around using a JSON file and importing it in Auth0, it is a method of user migration from WordPress to Auth0 which should still be working.
Hope this helps!
Gerald