Reddit mentions of Carlon B225R-UPC Switch/Outlet Box, Old Work, 2 Gang, Blue 3 Pack

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