Iowa works similarly as you describe. There are ways to cheat the early voting system piecemeal, but not so directly unless you got someone(s) on the inside.
What always bothers me is all the unsecured boxes of ballots that always seem to appear, and I assume disappear without a trace.
The previous county auditor here was known to play games with boxes of ballots years ago.
@Free VulcanI remember during one of the Obomber elections a state election official in a forgotten state (chemo brain) was stopped for suspicion of drunk driving,and when his car was searched there was at least one ballot box of votes in it. And was after the pols had closed.