I’ve got a mod that does something very similar to this called Compound Bow Plus:
To change the damage of an arrow you need to open the corresponding ProjArrow_ file. Search it for “damage” (without the quotes) and it will show both the “Direct Damage” and (more importantly) the “Damage Type”. This screen looks something like this:
If you open the DmgType_ file (click the little magnifying glass to find the one it is currently using in the content browser) you can set things like how much torpor it does. This is the settings I have for my Tranq Metal Arrow. I have highlighted the parts that relate to torpor in red:
Hope that is some help! Let me know if you have further questions and I’ll do my best to help.