Yes. You can buy extra processing minutes upfront from settings without changing your subscription. Your plan's included processing minutes are used first, then extra processing minutes are used after that.
If you have an active subscription, you keep all the current benefits of your subscription. Extra processing minutes only increase the amount of processing minutes available.
If you are on the Free plan and buy extra minutes, you still keep Free plan limits, except max input duration becomes 5 minutes and file upload limit is 256 MB. Compare plan limits on the pricing page.