No, you can’t add or remove days from a pass that you already have bought.

You either need to exchange the pass you bought, if you have a pass that can be exchanged, or you have to buy a separate ticket to use for the additional travel, As there is a cost involved with a possible exchange of the pass, it might be a good idea to check if it is worth to make an exchange or if it is cheaper to just buy a separate ticket.

NO, this is not possible. ONLY before the start date you do an exchange-give back (less fee) old pass and buy new one. You may have to wait 2 -3 monthes before money comes back, even though it says somewhere it should be quicker.

Others are more clever: they roughly plan the intended trips to make and for the shortest/cheapest they buy tickets when there-or maybe preorder with some chance on even lower price. OR re-plan better and the need for this disappears

LOcal/regional trains In IT are quite cheap-still. In some areas (Lombardia, Lazio around Roma, Umbria) there are also fairly cheap local passes for a few days that also include buses etc.