Once you have imported your styles, you could simply delete the original stylesheet leaving you with only the new one. The problem with doing that is you won’t know if the new styles work until you start using them.
WARNING! Freeway does NOT always correctly import all styles. Some will be not be correctly formatted after import.
Well, you could just reference a copy of the css you imported as a text file to see where things may have gone astray then add the missing bits in the styles editor. A lot of the missing bits can only be added with extended option by the way.