So there's two parts here: the stats data and the root template that uses that stats data.
Your root template can be created from an existing dagger, yes. That's the best way to do it, to save yourself some time. Just make sure you set the stats to your new stats data.
I noticed some unique weapons use their name as the item group (so WPN_FTJ_UNIQUE_BraccusTorch has an itemgroup called WPN_FTJ_UNIQUE_BraccusTorch). Look up the TreasureTable entry called "FTJ_FlamingPigsGuardian" in the Origins TreasureTable. You can see how they added the unique Braccus Torch to that. Dropcount is set to -1. Same thing for FTJ_Magister_Orivand. His unique mace has a dropcount of -1.
Actually, as I look more into it,
if you're not placing your root template in the world somewhere, I believe you need to add an entry to ItemProgressionNames.txt and ItemProgressionVisuals.txt. That's where you specify that the itemgroup "FTJ_Magister_Orivand_2HMace" = MyRootTemplateID.
Public\DivinityOrigins_1301db3d-1f54-4e98-9be5-5094030916e4\Stats\Generated\Data\ItemProgressionNames.txt
new itemgroup "FTJ_Magister_Orivand_2HMace"
add levelgroup 0,0,"All"
add rootgroup "91e5bd15-033a-418b-8fca-ea0bfe5922e6",""
add namegroup "FTJ_Magister_Orivand_2HMace","",""
Public\DivinityOrigins_1301db3d-1f54-4e98-9be5-5094030916e4\Stats\Generated\Data\ItemProgressionVisuals.txt
new namegroup "FTJ_Magister_Orivand_2HMace"
add name "Magister Orivand's Mace","Robust enough to put a stop to any conversation, Magister Orivand's mace fairly glimmers menace."
I haven't actually tried adding a unique weapon to a treasure table yet, so this is a learning experience for me as well.
So to get this to work for your mod, you'll need to create two text files in your mod's Public folder, under Public\MyModName_GUID\Stats\Generated\Data\, called ItemProgressionNames.txt and ItemProgressionVisuals.txt. There you add your unique weapon, like in the examples I shared.
rootgroup is the root template GUID of your item, levelgroup is the level range for that item to show up, itemgroup is the name you used in your weapon's stats, and namegroup is the namegroup used in the ItemProgressionNames file.
In the ItemProgressionNames file, it seems you can specify the name and then description of your weapon.