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
"str" (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/