You're doing this the wrong way round.

If you are going to be moving to Japan for a decade then you buy a one-way ticket to Japan. When you are then taking a trip from Japan to HEL or wherever you simply buy a return ticket originating in Japan thus eliminating the (increased fare) issues that can arise if you want to stay over, typically, 90 days.

And then 10 years down the line if you decide to go back to live in HEL you buy another one-way ticket.