There is no way to add anything to All Characters! However, there is a potential workaround.
1. Make a new folder. Call it something like "Characters".
2. Write in the description what you'd like to put if it were All Characters.
3. Move your other folders (Main characters & Secondary characters) into the folder from step 1. I.e. make the new folder their parent folder.
4. Set the default folder as your folder from step 1.
This won't be able to display all your characters at once, but it'll let you have a line of description when people open up your Characters tab.
I hope this makes sense! I can try to explain more if it doesn't.