Upvote:2
Yes -- once you have a visa, you're allowed to combine the conference trip with other business or incidental tourism in the Schengen area -- as long as you conform with with the length-of-stay and valid-until restrictions on the visa and you're still actually going to the conference as part of the trip.
In particular, you can travel through other Schengen states on your way to and from Poland.
(This holds for both single-entry and multiple-entry visas, except with a single-entry visa you must be careful not to follow an itinerary that will take you temporarily out of the Schengen area during the trip).