You'll want to use KCN, not NaCN. Potassium salts don't have *nearly*
the tendency to absorb water from the air (and thuse cake into unusable
lumps) that sodium salts do.
Cyanide has a distincyive odor of "bitter almonds". Apparently even
*non*-lethal amounts are smellable.
BTW, if there are members of a minor race on board, there's a *chance*
that they won't be affected. At least of of the respiratory pigments
(oxygen carrying compounds in blood) that is used by species on *Earth*
is unaffected by cyanide! Hemethyrin(sp?) is iron based, but not as good
as hemoglobin. And according to the Brittanica, it *isn't* affected by
cyanide!
Your *biggest problem is going to be the sensors in the life support
system. They'll detect almost anything out of the ordinary and seal off
that section of the ship. They *have* to be able to detect all sorts of
things, because you'd be *amazzed* at the compounds that'll turn up in a
small closed environment. (one of the earliest attempts at a sealed
environment had to be ended after only a week because Formaldehyde from
the plastics inside had accumulated to dangerous levels!)
If all you want to do is kill the crew, there are easier ways to do it.