but then I have to delete the Passwort value from the already imported users?
Could there be an option for each field to set up the behaviour?
For example only import passwort if its a new record?
You cannot set behaviour on each field but you can do two imports with one user file.
1. To import new users set Overwrite existing data to No on File tab in import settings. This will not overwrite or update existing users. Password is required field to import new users.
2. For existing users set password or password_crypt fields to skip template field so these fields are skipped on import. In this case set Skip new items to Yes on File tab in import settings. Skip new items will not import new users.