can I import private keys to a bitcoin core wallet from which once I already exported
The question is quite vague, but this is what I get:
You're trying to re-import a private key to the same Bitcoin Core wallet where it was exported.
If that's the case, it wont work (
nothing will happen) since that private key is still in that wallet.
Exporting a prv key won't delete the private key from your wallet.
If not, just carefully follow their guides (
above replies).