Currently I'm making some weapons that hold passive spells, which only activate on triggers (i.e. kill, damage_dealt, etc) and so these Wands do not have spell-inventories (as they are designed to just be used for melee, and the effects are passive)
I'm trying to make a way for the players to be able to add new passive spells to a weapon, but due to the lack of spell inventory there's no way to see the Spells included on a wand if traditional spell-learning methods are used.

I mainly wanted to use the Upgrade Slot feature, so players can add a spell they want by slotting an upgrade into their sword, and this will be reflected automatically by the Slot Lore, but unfortunately you can't add spells with slots, you can only overwrite them. If a weapon already had some spells and an Upgrade Slot has 'spells:' parameter with a new spell under it, the new spell will overwrite any previous ones. It also overwrites any previous Upgrade spells added too, with the most recently slotted upgrade taking precedence.

My suggestion is an 'add_spells' parameter for wand Upgrades. This would mean that the upgrade can have a list of spells (just like actually using 'spells' parameter in the config) which will be added to the wand's spell inventory, rather than overwritten.
(image included as an example)