There are no files that you need to "add the codes in". You have to create the files because they don't exist.
You have to develop a plugin (see instructions here) that includes an editor module. The editor module will allow Q2A to interact with whatever editor you want. In short, you will have to implement how both of them talk to each other.
You can use the code of the WYSIWYG editor as a strong reference on how to do this.