dumpprivkey "address"

The dumpprivkey RPC returns the wallet-import-format (WIF) private key corresponding to an address. (But does not remove it from the wallet.)

*bitcoin-cli help dumpprivkey

dumpprivkey "address"

Reveals the private key corresponding to 'address'.
Then the importprivkey can be used with this output

Arguments:
1. "address"   (string, required) The bitcoin address for the private key

Result:
"key"                (string) The private key

Examples:
> bitcoin-cli dumpprivkey "myaddress"
> bitcoin-cli importprivkey "mykey"
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "dumpprivkey", "params": ["myaddress"] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/