The quickest and easiest way would be something I'd probably do only if in a scenario like this. Otherwise I'd use a custom table or module.
You can modify that module directly in the database so you can edit those classes. By doing this you can then add the field through the UI and which will then show that field on the form.
Start by opening the Modules app. Then run this T-SQL code to find your CMS module ID:
where ResourceName like 'cms'
Then run this update statement to set the development mode to allow you to make changes:
set ResourceIsInDevelopment = 1
where resourceid = YourIdYouRetrievedInThePreviousStatement
Then clear the cache on your site in the System module. Next add the field to the Site class and save it. Run the last T-SQL statement and set the ResourceIsInDevelopment = 0.
Now you've added your custom site field and the module's security is set back so others can't modify it.
Again, for something very simple and straightforward like you're looking for, this works, otherwise I'd recommend a custom table or a module.