I am a beginner doing 40 day power meditation ..Instead of doing yogic breath in that session,can I do OTHER breathing exercises like pranayama,kumbaka,sithali etc etc
You can do other things also, but do not skip the yogic breath. If this is difficult for you, that is even more reason why you need to do it to get better at it.
Yogic breath is the basis for all of the other ones so do also include it. You are also free to choose whatever else you do but do not skip things as that might not be productive when building a foundation for other forms of practices.