create seperate dashboard by clicking content->Create a Subdashboard
<http://gits1sps/ARG/Portal/?Cmd=NewDB> and import all the webparts
for that dashboard.
but you can have only one document library in a workspace.you can delete
the existing dashboard by clicking - settings and delete button.

How to get standard or default weboarts like news, categories for a
particular Subdashboard?

Once you've installed your first workspace, go to every dashboard as
coordinator and visit the configuration for every web part and export
it. Create some warehouse of webparts somewhere and store them all.
Call the folder "Sharepoint Built-ins" and create a directory for each
web part.

Then your web part library has something that you'll need later on when
you either accidentally delete a web part you thought you wouldn't need,
or when you start re-using the built-in web parts on custom
subdashboard.

The next logical step is to create a Web Part Catalog of all Sharepoint
Built-in's so that you can easily select from the list.

I have imported on the newly created  dashbord News web part file exported
from Home Dashboard.

It is refering the same News html files stored in workspace.(which is
Obvious i know). But i want to know how to create/modify or where to place
my News (etc. like announcements) files/folders for this specific
Dashboard(or subdashboard for a whole portal).