These switches are on separate circuits. They must never touch or cross.
To really get the picture, it would be nice if there was a divider in that box.
It is not possible to put a receptacle on the lamp circuit because it is a switch loop.
What else is on the circuit with the disposal? If the hardwired loads on the disposal circuit total 50% or more of circuit capacity, then a receptacle is not allowed on that circuit.
Also if it serves any receptacle locations not in the kitchen, a kitchen countertop receptacle is not allowed on that circuit.
Edit: So you're OK. Son of a gun. This house is wired in conduit.
Turn the power off. This is harder than it sounds. We must shut off 3 things: First, the light circuit, because we'll be handling it. And second, the disposal circuit BUT-- it is part of a multi-wire branch circuit and we must also shut off the other half, because we'll be messing with its neutral, and the other half needs that neutral. If the disposal is a 2-pole breaker as it should be, we're all set; otherwise turn off frickin' everything...
First, get those rocks out of the box. What is with junction boxes and rocks anyway? You really, really don't want rocks falling down into the conduit pipes.
Second, note the lower left conduit, and the upper center conduit. Those are involved in this circuit. Figure out where they go in each direction. I would gather one direction (down?) goes to the garbage disposal and is quite close. That'll do. Push and pull the wires to make sure they are connected.
When you identify the black wire that goes from the switch to the disposal, mark it with red tape near both ends - this will be a timesaver later.
Guesstimate how far that distance is (in the conduit) and hit the store for
- 12 AWG white solid THHN/THWN-2 wire, about 3 feet longer than that.
At the disposal, unhook the white wire from the disposal and straighten it. Lay it alongside the new white wire for about 6". Lash them together with electrical tape starting at the wall and before they start to overlap. Leave the tail end of the tape so you can unravel it later. You'll be using the old wire to "pull" the new wire through the conduit.
Now try pushing the wire(s) up the pipe, out of the disposal box. Likely this will only go so far (it'll be stiff, it's solid).
Then, at the switch box, grab that white (between top center and bottom left conduit) and pull it out slowly, making it draw wire from the disposal side, so it sucks in the new wire. If it hangs, tease and twist it, don't brute-force it or you'll separate the wires and really be up the creek!
Leave about 9" dangling out of the disposal junction box. Reaattach this to the disposal wiring, and button all that up; we're done at the disposal.
Now, back at the switch box, you should have some extra white wire. Cut that off so it sticks out about 6" beyond the end of the wall. Get ready to make some pigtails; for black pigtails wrap the white wire with black tape.
Hey, why are we pigtailing neutral? Can't we just use the 2 screws on the receptacle? No. See that purple wire? That's why pigtailing is mandatory. It's surely the other half of a Multi-wire branch circuit and you must be able to remove this receptacle without severing neutral to the purple half of the circuit.
Hey, why are we pigtailing hot? Can't we just use the 2 screws on the receptacle? Sure.
Take your new (GFCI, surely) receptacle. Put a 6" white pigtail on the silver (LINE) screw. Put a 6" black pigtail on the brass (LINE) screw. Leave the warning tape on the LOAD terminals.
On the old disposal switch, remove the black wire that isn't tagged red, and replace that with a black pigtail.
Join all 3 black wires with a wire nut. Join all 3 white wires with a wire nut.
Plug a coffee grinder or something into the GFCI. Get some rubber gloves on, and turn the power back on. Carefully test that the switches work, GFCI powers up, and coffee grinder works.
Awkward as hell, but a very, very useful intermediate checkpoint.
Turn the power off.
Now that switch.
The switch will be delivered to you with a common "hot". You will need to either "break the tab" or something. Use a switch where the separation between switches is clear. I've seen switches where it's very confusing; those will backfire on you. You must break off the tab before you start.
By the way, if each half of the switch has 2 brass and 1 black screw, that's a double 3-way switch. It isn't wrong, but completely ignore one of the brass screws in each section. If you don't like which way the switch throws, switch to the other brass screw.
Move the two wires currently on the disposal switch, onto one switch on the dual switch. Doesn't matter which. Stop. Go no further.
Now turn the power back on, don the rubber glove, and test whether that switch actually works. If it doesn't work, you are very lucky to have caught it now.
Turn the power off.
At this point, there should still be 2 empty terminals on the new switch. Move the 2 wires from the light switch there.