can i increase range of my xbee s2c module by connecting to external antenna of higher dbi ?

I have two s2c module which has a range of 1200m LOS but i need to track a vehicle on a race track which has a radius of 1500m. Is there any way to extend the range of two module by using external antenna.

ONLY if the country you are operating in allows you to. That is, the module would be certified for a given output power and Max gain. You can only use what it has been certified for in that country.

Either use 2 very good high gain aerials or place a RF booster on each device. As your on a race track these are generally out in the sticks, so interference should not be a problem. Otherwise go for the 900mhz version as they tend to have a longer range.