That's stored in the publishedon Field (and/or the createdon field). Maybe there's a way to set that field before the import.
One thing to watch out for is that if you later re-publish the Resource, the date will be set to Today.
How are you importing the Resources and what platform are they on to begin with?
As a last resort, you can edit the publishedon field in the front end with NewsPublisher. Hopefully, it won't come to that.