If I recall it correctly, the Exterminator mod can be added to any weapon, except pistol or thrown.

In the description it says "... instead of firing your weapon you can fire the Exterminator...". Right?

But shouldn't this upgrade be limited to ranged weapons? Or did I miss something?

It would make sense because adding 1kg to a melee weapon would seriously unbalance it...

Our Priest has an Exterminator in the hilt of his warhammer. Comes in very handy against hordes of heretics. I think it is very stylish .

Yeah, building a one-shot flamethrower into a melee weapon isn't going to be that hard, and it is quite thematic for Dark Heresy/Warhammer 40k in general.

While sure, it may seem unbalancing, it's probably built into the weapon in a way to minimize such an impact; not to mention the utterly ridiculous weights of weapons anyways. Your average bolt pistol weighs as much as a modern rifle, your average plasma pistol weighs more, a 'basic' plasma gun weighs forty pounds. Your 'average' one-handed sword weighs six pounds (the weight of a greatsword). An eviscerator weighs 33! The weapon weights for anything but las-weapons and some of the Heavy weapons are ridiculous, in all honesty.

Back in the day, the first "Exterminator Cartridge" was attached to an Eviscerator, as standard, for Redemptionist Crusaders in Necromunda (or at least, that's the first I ever heard of them). It was only later that it became a weapon mod that was applicable to other weapons. As MijRaj points out, compared to the massive weight of a two-handed chainsaw of doom, a little one-shot flamer isn't much and I can quite easily see the bulk of it being built in to a weapon, rather than just being bolted on, which would ameliorate any unbalancing effect.

Okay, I never really checked the actual weight of a real weapon and it does seem that those of the 40k universe following FF is a little too much rounded up.
If a trained human needs two hands to wield a 2-3 kg Zweihänder than for sure making a chainsword 6kg and one-handed is a little over the top.

So yeah that Exterminator might not need to weigh 1kg but I honestly don't see it easily fitted into the hilt of any melee weapon, it could be and extension of it. Because basically, the hilt is is an extension of the blade as those two need to be one for structural stability.

So yeah, for a warhammer, this can be done, as it isn't particularly known for it's balance but for blades, which are used to parry, I do think the balancing of the blade is key.

Anyway this is a future fantasy setting, so I shouldn't be focussing too much on realism but my question is more oriented on the wording in the rules. "...Rather than firing the weapon as normal, the wielder may discharge the exterminator cartridge as a Half Action, ..."
As I do agree that is is an awesome cool sight to be spewing fire out of your warhammer or chainsword. I can't imagine how I would 'fire' my chainsword as normal ;-)

Depress firing rune > aim chainsword at enemy > ??? > watch as the Sororitas fawns over you over the charred bodies of your foe

That is how you fire your chainsword.

It would make sense because adding 1kg to a melee weapon would seriously unbalance it...

Even forgetting that the weight in DH means nothing at all, adding weight to a melee weapon will only unbalance it depending on where you put said weight.

I personnaly like the concept of a melee weapon with exterminator pack.