Is it possible to list open ports of a server, which is not accessible from my computer (running on Windows) , but is accessible from a computer to which i can connect via SSH?
This will only work if the computer you can connect to has NMAP available to it. NMAP is generating all kinds of traffic (TCP and UDP), and these would have to be tunneled over SSH. NMAP sends out lots of "bad" traffic, and I wouldn't expect SSH to tunnel it all.
The best thing to do is install NMAP on the remote computer and run it there.
However, only do this if you have legitimate permission to do this. If the remote site (to be probed) is someone else's and you don't have permission, it would be a violation of corporate policy and of local and federal laws to improperly access the site using NMAP.
Accessing a node is one thing. Accessing a list of open ports on a node is another. However since you mention nmap you can do
there's also this option or install nmap on the other computer