Can I add tickets to my Navigo Easy card online or by using the Vianavigo app?
Theoretically, yes the Vianavigo phone app can be used to charge Navigo passes (page in French), including Navigo Easy and any ticket it supports such as Ticket t+. But there are several reports of the app not working properly for charging Navigo/Navigo Easy cards with Android phones (on the Google Play store listing reviews). So I would be really wary of depending on the phone app for buying/loading tickets/passes onto a Navigo card. Apparently the app has better success on iPhone.
Regular Navigo passes can be reloaded online now via the Transilien website, but not Navigo Easy passes. Although, loading Navigo passes online still requires a visit to an automated ticket vending machine in order to update the physical Navigo card. So one can pre-purchase a week/month subscription for their regular Navigo card, but would still need to use the Navigo readers on vending machines for applying that purchase to their card.