During my wordpress plugin, I'm looking to get their email list of groups available in most the accessible network website.

Is it feasible without writing SQL query?

In my opinion you should use: get_all_category_ids()

The function is situated in wordpress-includescategory.php

here's more information: http://codex.wordpress.org/Function_Reference/get_all_category_ids

I acquired the solution. when we all of the blog id's in a wide array $blog_id_list , only then do we can perform:

foreach($blog_id_list as $blog_id)
    get_categories(); //use it as required.