You cannot convert an Electrum seed phrase to a BIP39 seed phrase, because in the process of deriving the master private keys, both algorithms use a different salt. This means, you cannot use Electrum seed phrases to generate BIP39-phrase addresses.
However, it is possible to derive Electrum addresses from Electrum seed phrases even outside Electrum. There is a short guide here even though it is for testnet, it also works for mainnet just by switching the coin type.
To quote it:
Download bip39-standalone.html.
Go to line 28611 and replace “return h == nh;” with “return true;”.
Go to line 28639 and replace “mnemonic” with “electrum”.
Open bip39-standalone.html in your browser and paste your seed phrase.
Select “Bitcoin – Testnet” from “Coin”.
In “Derivation Path”, select BIP141, P2WPKH and use: m/0’/0
Note: To get P2PKH address use the m/0 path, m/0’/0 is only for Electrum’s segwit P2WPKH.
