It is 100% possible, but that doesn't mean it happened. Ballot machines are kept offline for this very reason - its why the counts aren't instant. Additionally we keep paper ballots for the purposes of audits and hand recounts. It could be totally electronic, but we (and most other nations) have resisted that for this exact reason.
To my knowledge, every location where it has been requested has hand recounted. So, while I don't think there was fraud, there was time before the states certified to look into that. AZ is going through an additional audit now.
Industrial Control Systems (ICS) are also normally kept on isolated networks or totally offline in order to prevent these sorts of things from happening. I, for one, am chomping at the bit to find out who dropped the ball on this one. My bet is they connected it to allow real-time analytics and adjustments without having people onsite. I bet there is a PowerPoint slide somewhere where someone talked about all the money they were going to save by not having to have people go to each location.
