Can the campaign be ‘extended’?

Yes, of course, and not only is it possible, but also necessary! 

The ordered volume of reviews can be evenly distributed throughout the campaign period! 

Moreover, for new apps distribution is extremely important, since it won’t do any good if on the second day after launching the app in the store it will already have 50 reviews. This is dangerous, because you risk getting a warning from the app store that you are engaged in prohibited activities. Be careful and attentive when placing an order.