Deepika...you need to go through the Factories Act and more importantly the logic behind different type of leaves. The purpose of casual leave is to give short (1-3 days) and immediate leave to employees. Hence, Casual leaves cannot be:
1) Carried Forward
2) Encashed
3) Added with any other type of leaves.
However, one can also take half day casual leave.
Usually companies gives 12 days of Casual Leaves (where there is no difference in casual or sick leave).
Earned Leaves or PL's have different purpose. PL's can be anything above 20 days (As per Factories Act, companies are expected to pay one day earned leave for every 20 days of work). Some companies also give 30 days PL.
Employees can accumulate their PL's. They can encash their PL's.
I hope this will help you.
Regards
Sanjeev Sharma
(Blog:
http://sanjeevhimachali.blogspot.com/)