Hello,

I didnt find any mention of the SK capability in the firing and attack sections of the rules. So i dont know how it should be applied (in MdE the SK level of the units is added during combat).

Any idea ?

Yes, it is added in the same way as in MdE, just phrased differently, in the Infantry v Infantry only sub-section of the Testing to Attack section on p.16, col.2: "Skirmish differential + or -".
 

So, in Gd'D if an Austrian Batallion is 1 BW from a French Batallion, exchanging pleasntries... the Skirmish value has no impact?  They only count when the unit decides to charge into impact?

Just so that I can wrap my brain around that, does anyone know the logic behind that?

Volley fire only is a seperate phase. There is no Skirmish phase in the rules. The Skirmish capability is abstracted by assigning modifiers. Those modifiers and other effects are considered during close combat. So the game does have a Skirmish value impact. It's just not calculated until crunch time.