pH value of a buffer solution of HClO and NaClO So,
I have 200mL of HClO 0,64M.
I mix it with 0,1mol of NaClO.
How should I calculate the pH?

My question is about this: should I keep attention about changes made to the solution volume after adding NaClO?

So far this is what I've done.

Considering: 

$\ce{NaClO + H2O -&gt; Na+ + ClO-}$

With n(NaClO) = n(ClO-) = 0.1mol,

I calculated the molarity of the conjugate base:

[ClO-] = 0.1mol/0.2L = 0.5M.

Then I applied the Henderson-Hesselbalch equation:

pH = pKa + log([ClO-]/[HClO]) = 7.53 + log(0.781M) = 7.422

In this case I didn't consider the variation to the solution volume due to the addition of NaClO. So, is this correct?
You're close.  Everything is correct, except that when you take the ratio of concentrations in the H-H equation that ratio is not in moles.   It's just a number, because you divide moles by moles .  Rule of thumb: logarithms and exponential should never involve anything with units.  So don't include the molar unit under the logarithm and you're good.