I've read Qualtrics' tutorial on Preparing a .TXT File in Advanced Format for Importing, but it doesn't mention the format of a slider question, or any of its "specialty questions" for that matter.
Are sliders supported for advanced formatting? If so, what's the format for them in a .txt file?
Unfortunately, you can't. You can only import the question types in the instructions you linked to. Sometimes, you can pick the closest type to the one you want, then change the question type in Qualtrics and it will keep the choices you imported.