Dynamically creating a reactive array in the Vuex's state

Have a look at Reactivity in Depth - Change Detection Caveats:

Change Detection Caveats

Due to the limitations of modern JavaScript, Vue cannot detect property addition or deletion. Since Vue performs the getter/setter conversion process during instance initialization, a property must be present in the data object in order for Vue to convert it and make it reactive.

But you say you are adding an array dynamically:

I'm trying to dynamically add an entirely new field of array type to the SST and make it reactive at the same time. Possible?

From the docs (bold is mine):

Vue does not allow dynamically adding new root-level reactive properties to an already created instance. However, it’s possible to add reactive properties to a nested object using the Vue.set(object, key, value) method:

Vue.set(vm.someObject, 'myArrayName', [1,2,3]);

Which should help you making your array reactive.