Currently, Stellar Network has maximum 100 Billion lumens. I want to ask if it is possible to add more lumens to existing Standalone (private) Stellar Network?
Recently lumens were removed from circulation. There will now only be 50 billion in existence. Read SDF's Next Steps for more info:
All told, there are now exactly 50 billion lumens in existence.
Adding lumens would be difficult. It would require a core protocol change. So while it may be "possible" it would be very unlikely, as you would need the Stellar validators to vote in favor of that change.