Hi Chenna,
Based on the scenario you've described above,
Q1) Can we federate Server with Portal?
A1) Yes, you should be able to federate your Server site with Portal for ArcGIS.
Q2) Portal need to be authenticated with Active Directory. Is it doable?
A2) This depends. You would need to ensure that Portal for ArcGIS can access your Windows AD. You would have to open some ports to the DMZ, which may or may not be acceptable for your security requirements. In my experience, this is probably NOT a good idea as it would likely be a security concern.
Hope this helps,