The password generator, set to passphrase, has a neat option to change the word separator from the default “-” dash. It looks like any character is allowed except for empty space, which seems to me to be the next most-logical character people would want. Empty space is often completely fine for use in passwords and user names.
Note that you can enter a space in the word separator text field, it just doesn’t register as a character, so it’s also kind of a bug.
The space has already been supported. You can replace the default dash “-” with a space.
The disadvantage of the space key is that it makes a different sound when you press it so an adversary can know the length of your words.