Hi @Little - no, it isn't possible to do that.
If you were getting a value from context, then you could use a JSONata expression, as I described in this topic - How to use msg.payload as an index in a change node?
But you want to set a value in context... there's no convenient way of doing that in a Change node today. You'd have to use a Function node:
flow.set("store.mode_"+msg.payload.index,msg.enabled);
return msg;