How to import related products in normal import process? How can I achieve this? Is there any method available for import related products?
First, yes this can be done with Magento's Import / Export without the use of any extension.
Second, I would recommend taking one product, adding some related products to it in the Magento Admin, saving the product then exporting to CSV (System -> Import/Export -> Export).
You will notice two columns;
You will need to fill in those columns with the appropriate SKU. Please remember that each related product will need to be entered in a separate row (see screenshot).
This can be also done for crosssells and upsells.