Hi Lexima
Yes you can add or modify the KB, You said you can use the default KB, so in order to deploy your own you have to click on add source then select the option that you want ex- URL, file, etc, in this way you can add your KB, after that don't forget to deploy your knowledge base.
For your understanding, I have attached a pic and go through the documentation below for more info
If you find this answer useful kindly accept thanks much.