If you're trying to come across as more confident than you really are, look around you at the real extroverts. Put your extrovert outfit together in advance, adding as much color as you feel you can comfortably carry off. You might find that a new suit in a vivid color makes you feel energetic and outgoing. There are many ways to add color to your look without becoming overwhelmed. Experiment with contrasting scarves, colored earrings, and even a new, brighter hair color.
Colorful people attract more attention, and are far more likely to be perceived as extroverts.
Take the time to study the social behavior of the extroverts you work with. You're likely to see that they maintain eye contact when they are engaging someone in conversation, they smile and laugh a lot, and are not afraid to offer their opinion. You can start to emulate this behavior little by little - try increasing the amount of eye contact you make with your colleagues, and consciously begin to incorporate the act of smiling into your daily social intercourse. People will respond in kind, and start to see you as more open and approachable. There's no shame in having some conversation starters prepared for plugging those social gaps.