You can create a staging table in your domain model to hold all of the user information and user roles. Then you can use the import to excel function on this staging table, and then run a microflow after that retrieves all records from that staging tables and creates account objects and associates those accounts to the correct user roles.
Here is a quick example of what your microflow could look like.
You would retrieve all records from your staging table, iterate them and create a new account object. Then using your staging table retrieve the specific user roles you want and associate those to the new account.
If your users will have multiple user roles. You may want to create a separate staging table for user roles. Where you can retrieve all user role staging table objects, iterate them and retrieve a user role and then retrieve the an account based on an attribute in your staging table.
Hope this helps!