The export functionality doesn't allow you to filter the players, except for a specifying maximum number of players to export. So the way you would have to do this is to remove all non-bought-in players from the tournament and then export them. Specifically, press the "Remove Players" button, then click the "check unpaid only" link, then press OK to remove the players. Then export. This won't affect the running tournament because unpaid players aren't part of the tournament. But if you wanted those players to remain "added" to the tournament, this might be a problem.