Let us say we have a button component (I will call it btn_c1). I make an instance of the button component and then turn that same instance to a component as well (I will call it btn_c2).
That’s right, you can’t add a property to elements inside a nested instance, but you can show nested instance properties at the top level of the component. The text property of the component you need to add to “btn_c1” as you did in the next step.
Now select your “btn_c2” component set, under “Properties” click “+” and select “Nested instances”, then check the instances whose properties you want to show.
Create an instance of the component variant “btn_c2”, and check its exposed properties in the properties panel.