So from the Hunter's Point subway station, you would walk south one block on 21st St until you get to this underpass, then follow the sidewalk underneath to a footbridge on the other side:
Once across the footbridge, you would walk another block south to Borden Ave, then one block east to the stadium site. Here's an image looking north from Borden Ave along 21st St which gives a clear view of that footbridge:
I don't think existing infrastructure of the Hunter's Point subway station nor the footbridge between the station and the stadium will be sufficient to handle thousands of people for an event. Both will need upgrades.
The good news is the Vernon-Jackson station is almost equally as close, and has a much cleaner walking path to the stadium. The bad news is that station isn't the largest either - it's the nearest station to my apartment that I take everyday.
I think the Vernon-Jackson station is capable of being a secondary station serving those who wish to visit the bars/restaurants in the area before the game, but the Hunters Point station and the pathway from it to the stadium site will need quite a bit of work.