You don't need a book to offer an open parlay.
A parlay is just that, one bet and payout parlayed into another.
You're only issue with simulating an open parlay would be the bet limits you run into.
To simulate an open parlay, make your first bet.
If it wins, then take the Total Payout, that's original risk, plus the winnings, and then bet that on the next play. If that wins, then take that Total Payout, and bet it on the next play.
This is the same thing as an open parlay.