Adding new items to treasure tables is just "I_NameOfYourWeaponStat". Make sure you make the ItemGroup for your unique weapon "EMPTY", or it may show up as randomly generated loot.
Also, you should probably add "_Dagger" to your "USING" field and then tweak the stats, not copy it directly. The stats with a underscore in the name are meant to signify base stats that other stats will inherit from. So in your case, a better naming convention would be to name your dagger something like "WPN_UNIQUE_MyDaggerName". Then in your treasure table, add "I_WPN_UNIQUE_MyDaggerName".
