Thanks CyberJock!
Actually i have opted this issue in my Masters Thesis that what are the parameters and requirements to enable High availability feature in Free Nas, i know code does not exists thats why i need some guidelines that how do i contribute by writing the code and enable the feature.
I research on internet and came to know that many of other freebsd distribution support High availability, so i need some direction that from where i can start writing code/module that integrate High availability feature in freenas.