I think it would be very easy. Scott and Foster would be introduced as supporting characters. (Could be.) Just like Howard Stark or Tommy Lee Jones was in Captain America, or Kat Dennings was in Thor, or maybe Roady in the Iron Mans. They are there, we see they work at the lab, Pym is the leader and the boss. And subtely we see a little about them. Like Scott Lang has a young daughter named Cassie. Easy enough to reveal something like that. Have a photo of her on his work station, maybe the one of him and her that they reference in the comics lately:
And maybe have him mention her, like in a scene where he and Bill or Janet are talking and he could say something as simple as I have to get home to Cassie. Something like that.
He would in no way be the star, or even the sidekick/support character, but he would be a supporting background character who would later become important.
It occurred to me that if they didn't include him it might be easy to forget about him too, like Rick Jones in all the Hulk movies.