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Assuming that it's engines are incapable of dying out at 100km altitude, would mere addition of airfoil area enable a commercial liner e.g. B787 to reach that altitude?

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No. The principle of the aircraft – why it doesn't fall - is the aerodynamic force, the so-called lift. It depends on the existence and substantial density of the surrounding air.

However, 100 km is in the ionosphere. The ionosphere starts about 85 km above the ground and the density over there is about $10^{-10}$ kilograms per cubic meter (that's because the density and pressure are decreasing pretty much exponentially with the altitude: the Boltzmann distribution)

http://www.wolframalpha.com/input/?i=density+of+air+in+ionosphere

It's about 10 billion times smaller than the density near the Earth's surface. To compensate this decrease, you may be forced to install 10 billion extra wings. It's simply not possible.

All man-made machines at this altitude – above the normal parts of the atmosphere – are powered by thrust, like rockets. The very-low-density air in the ionosphere may still sometimes cause some problems, friction, but it's not high enough to be helpful for flying.

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I take it in quite a different perspective (Not as good as Lubos).

As per the Lift equation, $L=\frac{1}{2}\rho A\mu v^2\implies L$ $\alpha$ $\rho A$, we could observe that the force of lift is directly proportional to surface area ($A$) of the wings. But, it's also proportional to the density ($\rho$) of the atmosphere (at its altitude). As surface area increases, lift also increases as predicted by this Java simulator. But after a certain altitude, we know that there's a decrease in density of atmosphere. This balances the Lift force accordingly as the effect caused by $A$. But, there are also other things mainly weight (g-force) of the fuselage, wings, etc. to worry about. Hence, Max altitude reached by air-liners remain almost in the range 45,000-50,000 ft.

Here's a discussion on the same topic..!

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