Using an incremental extract would be my suggestion.
As Toby said, when you do a refresh you get the "latest and greatest" data. An incremental refresh would allow you to "add new data" to what you already have, while keeping the old stuff around.
If you're hoping to use extracts as some sort of a "historical snapshot" for permanent archiving of your data, it's probably not a great solution. Extracts are meant to speed up report execution vs. act as a classic IT historian mechanism (for compliance, etc)