Not with that attitude‡. A normal toaster uses about 1,000W, and a normal USB port is capable of 2.5W, which is 400 times less than the need.
First we make it a one-slice toaster, which cuts the requirement to 500W. We then apply advanced design and engineering principles to make the toaster more efficient. This may include an enclosure that better traps heat (but allows moisture to be drawn away from the bread). Or more carful placement of the heat elements to ensure that no heat is being wasted out the slot. Parabolic reflectors behind the heating elements can focus more heat energy directly to the bread slice. Lasers may even be involved.
All of these tweaks will drops the requirement to, say, 250W.
Still a ways to go, we decide to accept a longer toasting time in exchange for lower power requirements. Instead of heating elements covering both surfaces of the bread slice, we decide to have a single element on a track, moved by a stepper motor. This will toast the bread in sections instead of all at once. Think of the difference between a traditional camera and a flatbed scanner. This allows us to use a fraction of the power. Call it 50W. The toasting time will increase to 12 minutes, but if people like crock pots, then they'll love this.
Add 10% for the rest of the device and we'll need 55W.
So now we're at about 20x our ability. But wait! USB3 is common enough, that we can require it. USB3 can supply 4.5W, making our gap now 12x. There isn't any more power savings I can think of, so now we'll cheat by placing an onboard battery in the toaster/keyboard. The battery only needs to output 55W, so a single 20A lithium-ion cell will work. If we're estimating a 12-minute toast time, that's 11 watt-hours. An 18650 battery cell can provide roughly 11 watt-hours.
For only $129.99, you can have a keyboard with all the convenience of your kitchen. (Cherry switches are extra.)
‡ Yes, you're still right, it can't carry the power to toast bread. But the energy is there :)