First and foremost, the lack of any import feature within a current Webflow site is probably necessary. You can export, but can't import... this is hopefully the feature to help make this a reality:

1. Allow the ability to import an existing Webflow site into an existing one (and adjust classes for the import by appending a "-2" or "-import" to the end of the class)

OR ANOTHER APPROACH COULD BE....

2. When you copy/paste over from another Webflow project/site, that any class from that is maintained properly and does not adhere to the existing project/site (or better yet - you are given the option to keep the class as is or create a new one - which you could use the nomenclature adjustment as noted in 1. above).