Using your Operator console, you can edit your Surveys. Click on Settings > Chat Surveys , navigate to the desired site and queue, choose PreChatSurvey, Click New PreChatSurvey, add/remove/edit unwanted fields, then click on Replace current survey.
First add your Survey fields to correspond with the variables you're about to add in the PrePostChat.js.
You can automate the PreChat Survey and include the Visitor's username, or customer id number, or any other custom variables you might be using on your website.
You will need to go to your AgentInterface\Scripts and edit the PrePostChat.js'RenderSurvey method to pre-populate the chat.
On the server side, just navigate to C:\Inetpub\wwwroot\SightMax\AgentInterface\Scripts and open the PrePostChat.js in your favorite text editor.