Is it possible to host a server with the port closed but can be joined by other client?

BTW, I'm not sure how it called, UPnP? NAPT?

Well, because sometimes when port forwarding is not available and still wanted to play together (or test) with friends. At least, it is an option for the private group.

vpn -> like "Hamachi" but you should trust your testers before leting them in to your network
(firewall up + stop shared folders etc)

or you open the port + forward and your loginserver filter playerstatus (gm , tester , player)

or just create gameserverlist server in www:

client 1 start server --> force entry to the gameserverlist
client 2 find server --> search entry on gameserverlist
gameserverlist manage the connect

https://en.wikipedia.org/wiki/TCP_hole_punching
https://en.wikipedia.org/wiki/UDP_hole_punching

RakNet should have a NAT punching implementation