I want to run my WP frontend as
www.example.com and my backend (i.e. wp-admin) as
user1.example.org, is it possible?
The reason is I want to host lot of sites in a single server which has only 1 IP and I need SSL support for wp-admin (i.e. *.example.org)
There’s no need to do it the way you mean. There are ways to host multiple SSL websites on a single domain both with Apache and Nginx, and it’s much easier to implement than your idea. Check out these tutorials: