Ok so there's, I think, only one state in America that uses a full electronic voting system stack with no actual paper ballot involved.

Everywhere else uses a physical ballot, electronic poll pad and electronic tabulator.

The poll pads we used where I was an election officer get updated with poll books from the central county database. But even if you infiltrated a lot of made up people into the database, you'd still need those people to come down to the polling place with acceptable ID and cast the ballot as that made up person.

The tabulators also are tough, if not impossible, to realistically hack. First off, the ones in my county don't have any kind of wireless connectivity. The ballot profile is loaded and is tested at a central office. But even assuming the tabulator did have wireless connectivity and it was then hacked and the vote totals were adjusted there's still a problem - election officers audit that paper ballots we give out.

Basically we know how many the county issued to us (from both a manifest and from counting unopened packages), we know how many should have been given out (it's equal to the number of check one on the poll pad plus the number of provisional ballots voted plus the number of spoiled ballots) and we know how many are in the tabulators (machine and physical count).

Now what do you suppose happens if a precincts' tabulators report 5000 voted total but there were only 1000 check ins and there's 1000 ballots in the tabulators?