Is it possible to step-up the output of a laptop USB port to 12 watts?
No.  This is basic physics.  There is no free lunch (or energy).

If the laptop only puts out 500 mA at 5 V, for example, then you get 2.5 W.  You could convert this to a different combination of voltage and current, but the result can't on average exceed the 2.5 W you put in (It is possible to get higher power out for short durations, but that's clearly not what you are asking about.  The average out still can't exceed the average in.).  Since no conversion will be 100% efficient, you will actually get a little less power out, with the remainder getting dissipated as heat in the converter.

For example, let's say you can make a switching power supply that is 90% efficient.  That means with 2.5 W in, you get 2.25 W out at some other voltage and current combination.  The remaining 250 mW will heat the switching power supply.  You could get, for example, 10 V at 225 mA, 24 V at 94 mA, 2 V at 1.13 A, etc.