No. A top-up gives you a new validity period rather than adding days to your current one.
If your plan has 10 days left and you buy a 30-day top-up, you get 30 days - not 40. The 10 remaining days are lost.
So there’s no way to buy extra time alone
Validity always comes with the top-up you choose, and it replaces what’s left. The only way to keep every day you’ve paid for is to top up on the last day of your current plan, or once your data is used up.