You could conceivably use the Chop feature to do this. Instead of busting players out, initiate a chop at the end (Ctrl+C from the Game Window, or press the Chop button on the Controls tab). The Chop feature allows you to relatively rank some or all of the chopping players, so you can rank the final 18 players, leaving the rest alone (which would make everyone else tie for 19th place, but based on your description it doesn't matter).
On the Chop dialog, the rightmost column is for Ranking. Everyone chopping will have a relative value of "0", which basically means each player is "even" and thus ties. If you change a player's relative ranking, you'll see the actual ranking(s) change in response.
As an example, take a 10 player tournament. When the chop is initiated, everyone has a relative ranking of 0, so everyone's actual rank is "1st" because they all tied for 1st place. If you change the first player's relative ranking to 1 (or any value greater than 0), his actual rank will become "2nd". Since his relative ranking is now greater than everyone else's, he no longer ties and is ranked accordingly. Now change the second player's relative rank to the same value, and you'll see that player's actual rank change to 2nd as well. Change his relative rank to 2 and his actual rank will change to 3rd.
This is going in the opposite direction you need. For your situation, assuming you always care about the top 18 players, you would set the relative rank of the winner to -18 (negative 18), and the relative rank of the 2nd place finisher to -17, and the relative rank of the 3rd place finisher to -16, and so on, finally setting the relative rank of the 18th place finisher to -1.
It's a little confusing unless you see it in action. Try it with a mock tournament. Buy in 20 or so players, start the tournament, and then press Ctrl+C to initiate a chop. Once you start filling in values, you'll see the results and that should make it fairly clear.