Reddit mentions of Hubbell Raco 936 Ceiling Fan Box and Support, 4 in

Yup. One of these.

Do you have access above the ceiling? You’ll want to get a good ceiling fan support, this one is rated for closed ceiling access.

Make sure your support can support the weight of your fan. It’s not difficult to do. Good luck!

I've added a new fan/light combo (no fixture there previously) as well as added a new switch to an existing fan/light that ran off of one switch. Both places I used a GE 12724 Smart Dimmer for the lights and a GE 1273 3-Speed Fan Control for the fan. It works great in my opinion, you can manually turn the fan and/or lights on/off or I can use my phone (connected to my Rpi3 DIY hub running homeassistant).

Here's what you'll have to do, it's a little bit of work but if you're handy it's not a very difficult job. If you have access to the fixture from the attic this will be 100% easier.

Assuming that there is a single 14/2 Romex wire from your current switch to the light you'll either have to:

• pull another 14/2 Romex wire from the switch receptacle to the light fixture
  
  
• replace the current wiring with a single 14/3 Romex wire
  
  
• replace the current ceiling enclosure with an old-work fan-rated box assuming the box isn't ceiling fan-rated
  
  
• assuming that the switch is in a single-gang enclosure replace the switch enclosure an old-work 2-gang enclosure
  
  
• wire the fan and switches with the new 14/3 Romex or the dual 14/2 Romex wiring