How is Bitcoin Core 0.16?
https://github.com/spesmilo/electrum/blob/master/RELEASE-NOTES#L94
I strongly recommend simply sending the coins over to the core wallet in an on-chain transaction.
However if you want to convert the priv keys to a standard format then here is how you can do that:
On the console tab (view show > console if it doesn't appear) type this:
format,privkey,compressed=bitcoin.deserialize_privkey("")
Then to generate the key for importing into core do:
bitcoin.serialize_privkey( privkey, compressed, "p2pkh")
The above should output a key in the standard WIF. You may need to addwitnessaddress for this address in core's debug console.