In the ammo or attachment look for “Support Drag Onto Item Classes” & “Ammo Support Drag Onto Weapon Item Weapon Templates”
Look at the tranq arrow for an idea of whats going on there. Basically it changes the weaponbp depending on item type.
Here is another example
I don’t think there is a way to stack the attachments but what you could do is put the weapbp WITH the attachment as the “Ammo Support Drag Onto Weapon Item Weapon Templates”
For example on the tranq dart, if you replaced “WeapOneShotRifleTranq” with “WeapOneShotRilfeScoped”, you’d have a scoped one shot rifle that shoots darts. Only thing with that is that you would NOT see the dart being loaded into the gun. However, you could get around by replacing the bullet mesh with the dart mesh on the scoped version.