electrum seed is not bip39 so you can't use the coleman site to generate address specific private keys.
You can modify 2 lines of code in the iancoleman website and have it generate the keys from an Electrum seed mnemonic 
if you are knowledgeable and willing enough to take the risk and start modifying code then why not just use Electrum code itself? you can go to its repository and just take the part where the Seeds and key generation stuff are and use it to generate your private keys.