He could get it, but it probably wouldn't make any kind of difference. Nasus doesn't get a large stack bonus from getting kills with his Q in the same way that Veigar gets a large bonus from getting kills in general. Nasus gets a much higher percentage of his stacks from minions than Veigar does.
Basically, I don't really think this is the kind of QoL buff he really wants. A very small increase in stacks for fighting isn't going to change anything, and it's not going to make enough of a difference to feel good. I already feel no particular desire to specifically go for Q kills in a teamfight, so I don't see much of a need to fix that. This would be more akin to if Veigar got the 2 bonus AP from a Q stack every single time he got a kill or assist. I'd rather extra power be added in ways that would feel better than this. I mean, this wouldn't be bad, but I think we can do better.