generating seed phrases offline and which wallets support custom passprhases?

[acme89]

Ok, I've found some more explanations. Just it is too bad that Metamask and majority of other wallets do not support seed phrase customization.

https://getcoinplate.com/blog/what-is-a-bip39-passphrase-enhancing-the-security-of-your-crypto-wallet/

...Is there a limit to how long my passphrase can be?
No, there is no technical limit to the length of your passphrase. However, for the sake of memorability and practicality, it’s generally recommended to keep your passphrase relatively short, while still maintaining a high level of complexity and uniqueness. Furthermore, each wallet can have it’s own length limit so make sure to check if that’s the case. For example Ledger wallets allow 100 characters while Trezor 50 characters. Overly long and complicated passphrases don’t offer much added security, so usually 15-25 characters are more than enough...

[Pmalek]

Can we use any word of our choice as that additional word (i.e. a word that is not included in the list of 2048 words)?

You can use anything you like. It doesn't have to be real words at all. For example, Hgjdl=)87 could be a passphrase. You can hit any keyboard combinations you feel like. But I still recommend using something relatively normal because you don't want to find yourself in a situation where you or your heirs have trouble recovering your coins because of a complicated passphrase. The length is important as you noticed doing some additional research. When I set up a passphrase on one of my hardware wallets, I could only use X words because I reached the maximum character length.   

[nc50lc]

Ok, I've found some more explanations. Just it is too bad that Metamask and majority of other wallets do not support seed phrase customization.

It feels like our replies are getting ignored by you OP.

Four of the previous replies, including mine have already addressed that
and one part of my reply in particular suggests that it's not proper to use the term "customization" because it'll just cause confusion.
Use the term "BIP39 Passphrase" or "Passphrase" like what's used by every user in this thread and in the article that you linked.
The latter may be mixed up with with the wallet encryption password, in topics about wallet encryption and mnemonic, I'd suggest to use the former.

[Cricktor]

Overly long and complicated passphrases don’t offer much added security, so usually 15-25 characters are more than enough...

You should also take into consideration that you need to input an optional mnemonic passphrase to unlock your wallet when you use a hardware wallet where you usually have a very limited input interface. Having to input a rather long and complex mnemonic passphrase can be a pain with most of hardware wallets I know.

I don't consider online hot software wallets as safe enough to secure substantial amounts of coin value, therefore the usability of a wallet setup with a hardware or air-gapped wallet is of importance for me.