Yes. You may normally switch out attacks for combat manuvers, and the flurry of blows does not change that.
FAQ about full attack: Replacing Attacks with Combat Maneuvers
Any combination of a creature’s attacks during a melee full attack can be replaced by a trip, disarm, or sunder maneuver (any maneuver that says “in place of a melee attack”).
Any character can replace any number of attacks during a full attack with trips.
Flurry of Blows: At 1st level, a monk can make a flurry of blows as a full-attack action. When making a flurry of blows, the monk can make one additional attack at his highest base attack bonus. This additional attack stacks with the bonus attacks from haste and other similar effects. When using this ability, the monk can make these attacks with any combination of his unarmed strikes and weapons that have the monk special weapon quality.
The monk flurry of blows gives you an additional melee attack. You may replace melee attacks with trips. Therefore, you may replace the additional attack with a trip.