Yes, if you are changing plans, visiting more countries (lucky you!) or have a layover in an additional country, it's very easy to add countries to your itinerary.
Mobile app (iOS and Android)📱
1) Go into your Trip Settings (the cog icon in the top right).
2) Select 'Destination', and then search for, and select, the additional countries you are visiting.
Web 💻🖥
1) Select 'Edit Trip' (top right) on the Overview page.
2) Select 'Destination', and then search for, and select, the additional countries you are visiting.