I don't think there's any available poison that's sufficiently fast acting to be of use in Finland, and I very much doubt there's any historical indication of poisoned missiles (poisoned bait is a different issue, but I wouldn't be surprised if that wasn't used during the Iron age).
Oh, there's plenty of suitable poisons (see below). Question is if were they used, and if not, does it matter? This game is a fiction in a fictitious world, so why should it be limited to what we know about the history of real region, especially since we know not that much (before Sweden and Novgorod/Russian conquests?)
While we don't know if the natives of ancient Finland used poisons, we know that their neighbours Slavs (all three groups, including the ones who would later form Poles, and Rus') used poison arrows, Balts and Lithiuanians used poison arrows, Mongols and other Asiatic nations used poison arrows, and before them Scythians used very infamous poison arrows. If the action of the game is in the beginning of middle ages, then the Njerp could have been based on marauding Scythians (maybe after the fall of their empire), and they should definitely use poison arrows. It could be balancing to the game if they used it, actually.
As for what could be available, because it can be found in Finland and was used as poison by neighbours:
1. animal - viper venom was the main ingredient of the scythicon (toxin of Scythians) along with excrements, very deadly concoction, though it would work even better if infection (gangrene) was implemented in the game
2. plant - hemlocks - cowbane a.k.a northern water hemlock (Cicuta virosa), poison hemlock (Conium maculatum), both very toxic, effects can occur after 20 minutes if dose high enough, works similarly to curare; prefers wet places near rivers
3. plant - wolfs bane - varied Aconitum species, especially strong wolfsbane (Aconitum firmum), which is one of deadliest plant of Northern
Hemisphere, contains potent neurotoxin, also works similarly to curare, Japanese natives used it to hunt brown bears, but it was used in Central Europe and by Asiatic northern tribes too; it grows in mountainous areas
4. plant - false hellebores - varied Veratrum species, especially white hellebore (Veratrum album) and black hellebore (Veratrum nigrum), were used in warfare both in North America (by Natives) and in Europe and Asia (for example historically by Slavs and Lithuanians). This is interesting, because the same plants were used for the same purposes in completely different cultures, some of them were geographically close to Finland though.
Slavic princes after Christianisation began to ban use of poisons on religious grounds, but there still remain villages founded in this time which basically are named "poisoner's village", because their inhabitants were required to prepare battle poisons (as other villages were required to make iron, or hunt beavers, which is reflected in their names). Lithuanians and Balts are known to oppose Christianity, even after formal conquests, and they used poisoned arrows even till Renaissance (there are complaints to their Christian kings which describe this "unchristianic" practice). And they lived very close to Finland, so I wouldn't rule out a technology transfer. If not to natives, then at least to Njerps.
Also, poisons in real life take time to act. If used for hunting the idea is that the hunter follows the animal (or runs away from it if it's a bear) till the poison kills it, which may take hours. This would be perfectly in tune with present technique of shooting an animal with broadhead arrow and then following it. I think that poison arrows could be a home-made alternative to broadhead arrows, if available to the players (maybe only some cultures), or a valuable find when looting Njerp.
When used in warfare, it can debilitate the enemy rather quickly (cramps, loss of hand-eye coordination, sight loss, or even consciousness loss, like after hemlock), but is shouldn't kill quickly enough to fire-and-forget-and-select-another-target technique, more like should add penalties first. After all, it was more often used in guerilla style tactics than in open, huge battles.