whether mobile app in outsystems can be developed in already .net developed application?

Hello Vivek, 

I am guessing you want to ask, If one can develop a mobile app in OutSystems where the backend is already developed in .Net. 

If my guess is correct, then yes, if your .NET backend is exposing APIs (REST or SOAP), one can build a mobile app in OutSystems. 

If you can articulate your query/ requirement in more detail , then you can get a more detailed answer.

Hope it helps !!!