No, it's not possible to add more days or countries to your Pass. If you do decide to travel for longer, you can simply buy another Pass and add it to your phone.

Once an Interrail (or Eurail) pass has been purchased and issued, you cannot add extra travel days or extend its validity period. The pass is a fixed-term legal document, and the system does not allow for "top-ups." If your travel plans change and you find yourself needing more days while already on the road, your best option is to purchase a second, smaller pass (such as a 4-day or 7-day mobile pass) to supplement your original one. Because mobile passes are delivered instantly via email, you can buy and activate a new one within minutes of your first pass expiring. If you haven't started your trip yet and realize you need more days, you can exchange your pass for a different one through the official portal, though this usually incurs a €25 exchange fee plus the difference in price. For those on a tight budget, it is often more cost-effective to simply buy point-to-point tickets for short regional hops or use local "saver" fares for the extra days of travel rather than buying an entirely new international pass.