That dropdown is rendered by ItemBuilder.cs. If you wanted to add your own ItemActions to it, you'd need to edit that script in at least a couple of places: adding your ItemType to the ActionType enum at the top of the script, also adding it as a switch case in the AddAction method. There may be other places in the script that would need to be edited too.
Note that you can simply add/remove ItemType components to an item gameobject too, so in most cases there's no need to worry about the editor side of things.
To create a custom ItemType, you'd need to simply create a subclass of whatever type you want to inherit from. E.g. if you want to create a custom ItemType which is based on ShootableWeapon, then you'd create a class that inherits from ShootableWeapon (e.g. public class MyCustomShootableWeapon : ShootableWeapon
). Or to create a completely custom ItemAction, just inherit from ItemAction instead. I'd recommend taking a look through the base ItemAction class. Also the Flashlight class is a good example of a simple custom ItemAction (it inherits from UsableItem).