Hi Guenther,
Yes, this is definitely possible with our App. What we'd recommend though is that instead of letting the customer choose the additional days is that you automatically choose the variant corresponding to the number of days specified in the from/to datepickers. We've done this for a few sites now (here and here for example). The advantage of this is it prevents errors and allows you to offer different rates depending on how long the equipment is rented for. Please contact us if you'd like further info.
Thanks, Gavin.