Are you asking if it's possible to build a custom admin view against Tableau Server, then publish it to a separate Tableau Online site? If so, the answer is yes, but because your Tableau Server postgreSQL database is not in the public cloud, Online cannot connect directly to it. You will need to refresh your extract using Tableau Bridge.
I tried the bridge but i had problems with it.
but eventually the problem was IP permissions - my server had no permissions for its own IP