You can plan where you like but I don't think you can transfer the journeys from one device to another.

But to be honest, it's better not to plan with the rail planner app. It doesn't get enough updates to be reliable. It's better to use national websites or apps and for international planning, the DB planner or DB Navigator app is a good choice. It has the same coverage as the rail planner app (it also misses e.g. many Spanish trains and parts of Italy) but at least it gets 2-3 updates per week instead of every few weeks.