Yes you can. As long as you have a valid ticket for your journey, it doesn't matter where you board the train in. Also, as Lassi Uosukainen stated, Pasila is part of Helsinki and they're essentially treated as the same stop, and tickets usually aren't checked between the two. It's less than a 5 minute distance, so even if they DID check, they couldn't check everyone.
The only "source" I found was this interview in an article regarding another thing: buying a sale train ticket from place A to place B that stops in C, and getting off at station C.
The Finnish Railways former Vice President Commercial Lassi Rönkkönen commented on the matter:
– Se ei ole missään tapauksessa kiellettyä. Asiakkaat hyödyntävät
osavälejä näistä tarjouksista, Rönkkönen sanoo Taloussanomille.
It's in no way forbidden. Customers are taking advantage of parts of