I agree with the other posters - ankle weights stretch your knee joints in stopping the momentum of the weights with each and every stride, and as such are potentially dangerous. The knee is designed to be compressed, not stretched. There's a reason why "the Rack" was a medieval torture device.
A backpack or weight vest keep the weight close to your center of gravity, and are a much safer way of adding weight.
Ankle weights can be used safely for the slow and controlled movements of strength training, but not for the faster movements of cardio exercise.
In terms of tracking calories from additional weight, try the calorie calculator at www.caloriesperhour.com/index_burn.php
You can add 10 lbs to your bodyweight, and then enter the calories manually into Spark.