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

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

"key"                (string) The private key

> 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;'