Hello @Prasanth Gunnam,
Thank you for posting in our Q&A forum.
Based on the description, whether you can add a Windows 2019 server to the existing DC which is running on Server 2016? Or whether you can add a Windows 2019 Domain Controller to the existing DC which is running on Server 2016?
If you mean it is a Windows 2019 server, of course, you can, because:
Functional levels determine the available Active Directory Domain Services (AD DS) domain or forest capabilities. They also determine which Windows Server operating systems you can run on domain controllers in the domain or forest. However, functional levels do not affect which operating systems you can run on workstations and member servers that are joined to the domain or forest.
If you mean it is a Windows 2019 Domain Controller, I hope the answers provided by the two persons above are helpful.
If you have any question or concern, please feel free to let us know.
If the Answer is helpful, please click "Accept Answer" and upvote it.