I've searched for similar methods in this forum and reddit but couldn't find any, so apologies if this was already known. I found this out when messing around with venom pools that appeared when I was stabbing things with darts.
This requires 3 things. You need a water tile, a weapon with venom already on it (usually from subterranean animal people), and an animal or companion.
Take the venomous weapon, in my case it was a tower-cap blow dart with a helmet snake venom covering, and put it in your hand. Enter the water with it, and you will see a pool of venom form in the water tile. Normally, you can fill containers with an infinite amount of venom from a pool, but since it's in water it becomes laced with water, which is useless. In order to move it onto a land tile and purify it, you must move back and forth between the land and water tile until a pool of venom forms on the land tile because of the spatter spreading mechanic. This method of dwarven distillation concentrates the dilute venom in the water into a potent pool on land where you can either grab it with your hands or fill a water skin.
There are three methods of using this against your enemies. You may rub it on your teeth for biting, on your hands for scratching, or put it on a weapon. To get it on your teeth and hands, you must strip naked and drop everything (or the venom will only covers your clothes) and throw venom on a wall until a line of venom pools are created. Then you crouch (doesn't work with standing) with (s) and crawl across from pool to pool multiple times until the spattering and smears turn into coverings. A single pool will not work, you must crawl from one pool to another only, multiple times. I tested this with four pools. By pressing (e) you can see your body parts are now covered with venom, including the teeth and fingers. Biting something will transfer the venom into the enemy, you can see this by pressing (l) on the enemy and seeing if it transferred onto the body part you bit. The same applies to scratching. I tested this on a cave crocodile and the bitten and scratched body parts swelled up with blood, so I assume it worked. To poison a weapon, you throw the venom at your companion or animal and smack him bluntly (edge might cause blood spattering instead) on the part it landed on with your weapon. I tested this by throwing helmet snake venom at my gorlak companion and hitting him with the pommel of my large copper dagger until a covering of venom formed on the weapon. This might require multiple fresh venom coverings since I'm not sure if a full covering can form from hitting a spattering or smear multiple times. I then slashed the same cave crocodile, ran away, and waited until the part swelled up from the venom. If the weapon embeds, it ruins the venom covering instantly since it replaces it with a blood covering. However it's usually good for a few slashes, but not every attack is guaranteed to apply the venom.
Have fun.
P.S I also found out if your hands are full you actually CAN (g)rab venom directly from the water since it sends it to the backpack. If you try grabbing it with empty hands it doesn't appear in your hands for some reason. And if you try putting (i)t in a water skin it becomes laced water. So in case the back and forth centrifugation doesn't work (which it doesn't sometimes) try this instead. Also, since syndromes work much more slowly in adventure mode, you might have to edit the raws to get the most out of this. Also make sure to hit a different body part each time as the blood spatter from the previous slash tends to replace the covering. And drink any water coverings on you before rolling around in venom pools. Enjoy.