ToME WikiRavenRed: Just a minor sub-type of the Ammo Creation skill
Poison Ammo - Cost 4:Prereq Sneakiness@10, Archery@10. Free for Assassins @ Clev 8. Requires a potion of poison and "poisons" a max of ten arrows per use, giving them the appropriate brand (without any additional +s).
Useful, but not a gamebreaker.
GreyCat: First, assassins don't get the archery skill at all. For some reason I've never been able to fathom, they just get Boomerang-Mastery. They're designed as "sneak up behind 'em and slit their throats" type characters, not "shoot poison darts into their necks" type characters.
Second, while your idea seems useful, I don't think anyone's going to select it when Ammo Creation has lesser prerequisites and only costs 4 more points, and is dramatically more powerful. (Then again, you could poison all the merely-{good} ammo you create instead of throwing it away.)
Third, instead of hard-coding a maximum of 10 arrows per potion of poison, I'd just tie it to the Archery skill. Since you have Archery@10 already listed, you'd start out with 10, but at Archery level 20, you could do 20, etc. But that's assuming you gave it for free to someone who actually had Archery skill, but who for some reason didn't get or didn't want to spend points on the Ammo Creation skill....