I am wondering how to do fanout for a net i.e. GND not to add the vais manually and to have a better connection between different GND mixed planes in different layers.
PADS Router will do the fanout for a component; asking for the fanout for a net seems odd.
Check out 'Via stitch' and 'Via shield' in PADS Layout. That may be more of what you want. If not, add more detail to what you are looking for.