Welcome to FT! I don't know the Finnair rules specifically, but most programmes have policy of not allowing the same name/address combination to be used like this. I'm sure you could make some minor changes so that a computer couldn't pick it up. (Have you read the Finnair terms and conditions on this?)

If you are flying HEL-PRG-NRT, wouldn't the second flight be on JL? You may need to check earning rates on that fare, since I class (the cheapest business fare) earns only 70% mileage.

Edit 1: Out of interest, I read the T&Cs and couldn't find any reason not to do this. Good luck.

Edit 2: There are no non-stop flights between PRG and NRT/HND. Are you sure about your routing/airlines?