Adding Combo Switch to Existing Switch Loop
You don't.
When you have a traditional &quot;switch loop&quot; like that, you can't extend anything off it.
The essential wires you'd need for anything else to come off it, simply are not there.
A lot of people get confused because they see black and white wires and think &quot;those are the wires I need&quot;.  No.  The wires are black and white because that's how cables are made. In any given circuit, the black and white are reassigned to the needs of the moment depending on where it is.
On a switch loop, there is no neutral.  White has been re-tasked to be always-hot. Black is switched-hot.  No neutral, no circuit extension possible.