Of course you can. With each top-up, you can decide on your top-up amount again. You are never committed to one single bundle. Your most recent top-up will determine your calling rate.
If you top up with € 50, you’ll make phone calls for € 0.05/minute. If you top up for € 15 the next month, and you still have € 10 left from your previous € 50 top-up, the entire calling credit (i.e. € 10 + € 15) will be used at a rate of € 0.15/minute.
The calling rate outside the bundle is always € 0.20/min.