The existing base plan validity data vouchers are called data boosters and you can't boost something that your base plan didn't have in the first place which is why it's not applicable.
As of now the only option is to add the ₹175 data pack which gives 10 GB for 28 days. But that will make the plan costlier than daily data plans. So if you need the minimal data plans you can consider porting to Airtel or wait for Jio to lunch similar plans which they most likely will to compete with Airtel.