just create a standard electrum wallet with a fresh electrum generated seed. here's a guide in case the op needs it.
Additionally, it may also cause confusion if the user attempts to migrate to another wallet... and then wonders why their "BIP39 seed" doesn't work in a BIP39 wallet and shows as invalid! (Because it isn't actually a BIP39 seed mnemonic, it's still an Electrum "native segwit" seed mnemonic.)
If you want to use Legacy with Electrum, you're a lot better off creating a fresh Legacy Electrum seed mnemonic than attempting to "force square pegs into round holes."