1) If you are thinking of harmonics as sinusoidal waves, well yes, ALL waveforms are (can be described as) sum of harmonics. This is essentially the idea of the Fourier analysis.
The problem is that to exactly reproduce a desired waveform you need in general an infinite number of harmonics. This is for instance the case of square waves. So in reality you only use an finite number of harmonics if you want to approximate a square wave. BUT.. summing a large number of harmonics (with the correct amplitudes/phases) in order to produce a square wave is cumbersome, so the practical way of producing a square wave IS by quickly changing voltage (think of opening and closing a switch periodically).
2) This will not produce an exact square wave, because to produce an exact square wave you would need the circuit to have an infinite bandwidth, which is impossible to achieve. Essentially your circuit will always behave as generator of a theoretically exact square wave with in series a filter that will not allow the highest frequencies to be transmitted, and the impulse so produced will therefore be a "almost square" wave.
How good this is depends on what you require, i.e. if you want to generate a very precise square wave you need a more sophisticated "switch" (i.e. fast switching circuits, high frequency electronics). On the other hand if what you need to generate is going to be used in an experiment where you will only be interested/able to measure frequencies up to F, your "square wave" doesn't need to be exactly square, but only "almost square" i.e. containing frequencies up to F.
For some ideas about electronic circuits, check this book, it is an old but respected source.
EDIT: to clarify better the answer to part 2), you practically never produce a square wave (or any other waveform) by summing harmonics, but you use a special circuit that produces the desired waveform (search Wikipedia for "astable multivibrator" for a circuit generating square waves). And given enough resources you will be able to achieve a "as square as desired" (albeit not exactly square) wave..