Ok, Gentlemen, sorry for the late reply.
Here is the procedure, tested with electrumx-ftc and the core wallet version 19.3
Step 1:Open the electrumx gui go to wallet -> private keys -> export This gives you a list of your addresses, the type of private keys and the private keys. save the list as csv file
Step 2:open the core wallet gui go to window->console window in the console window, last line type importprivkey <private key from csv file>
The core wallet then will import the private key and scan the blockchain for coins associated with the corresponding address. repeat the last step for all addresses
That’s it :)