In Electrum, select the Wallet menu -> Private keys -> Export. You will get the private key of all addresses in your wallet at Electrum.





In Bitcoin Core, Open the Bitcoin Core Console (Window menu -> Console); in the input field, type importprivkey "privkey". Then the (Wallet) Rescanning window will appear.


If you want to import more than 1 Private Key, use the 'false' argument in the first 'importprivkey' command and so on, then in the last 'importprivkey', don't use the 'false' argument, example:


Then the Blockchain rescanning process runs after the last private key is imported.