I hope is not too off-topic but regarding "Custom Word extensions" to the seed phrase.
The custom word you add to your seed phrase is like an extra word. It's also called passphrase and some call it 13th or 15 word.
Take note that with any change to your passphrase, you will have completely different addresses and you won't be able to recover your wallet from seed phrase, if you don't know the passphrase.
It may worth mentioning that the passphrase doesn't have to bt a word. It can be any combiantions of words or characters.
How does this work? Because i guess that if i try to import a seed phrase to another wallet, let's say "BitcoinCore" i don't think they will support the extension of custom words i added to my seed phrase?
If you have added a passphrase to your seed phrase and you have to recover your wallet using a different software, you have to use a wallet that supports passphrase.
Take note that electrum uses its own algorithm for generating a seed phrase and if you have generated a seed phrase using electrum, you can import it in electrum itself and bluewallet.
You won't be able to recover your wallet from seed phrase using other softwares, regardless of you have added a passphrase to your seed phrase or not.
Also note that bitcoin core doesn't support seed phrase at all.