Buffer Solution with Strong Acid?
Buffers can be made two ways.

Buffers

Weak Acid/Base + Salt of the Conjugate Base/Acid

Let’s take acetic acid ($\ce{HC2H3O2}$) and sodium acetate ($\ce{NaC2H3O2}$).

$$\ce{HC2H3O2 + H2O &lt;=&gt; H3O+ + C2H3O2-~~~~~~~~~~K_a=1.8 x 10^{-5}}$$

Acetic acid has a small $\ce{K_a}$ value, meaning that it does not dissociate much. Conversely, sodium acetate readily dissociates as sodium is an alkali metal. By having equimolar concentrations of both weak acid and conjugate base, the addition of an acid adds hydronium, which reacts with the acetate ion to shift the equilibrium to the left, and the addition of a base absorbs hydronium to shift the equilibrium to the right. Either way, the hydronium concentration, which determines the pH, remains close to the same.

Weak Acid/Base + Strong Base/Acid

This relies on the same principle as above but supplies the conjugate base in a different manner. Continuing with our acetic acid example, here, this would be added, alone. Then you would add half of the moles of acid in strong base, such as $\ce{NaOH}$. This would neutralize any available hydronium, shifting the equilbrium to the right until there was the same half-and-half of acid and conjugate base.

Your Solution

Strong Acid?

Unfortunately, strong acids do not make for good buffers as they fully dissociate -- the equilibrium is so far to the right that it is very unstable.

Salt of the Conjugate Base?

$\ce{Na2SO4}$ shares no common ion with $\ce{HCl}$, so it provides no conjugate base ($\ce{{SO4}^2-}$ is the conjugate base of the second dissociation of $\ce{H2SO4}$). Even if you used $\ce{NaCl}$, it still would not work because of the reason outlined above.

pH?

If you're still interested, the pH of the solution above is easily calculated. As $\ce{HCl}$ fully dissociates, its concentration (0.1 M, as you say above) is equal to the hydronium concentration. However, the addition of another liter of neutral solution halves the hydronium concentration to 0.05 M. Therefore,

$$\ce{-log(0.05)=pH=1.3}$$

Hope this helps!