I solved it myself exploring all joomla virtuemart tables. I'm posting answer here may be it can help others. Actually the table to store related products is
#__virtuemart_product_customfields.
If you are not able to add related products from joomla administrator then insert rows to this table. You need to add only 3 fields i.e
1.virtuemart_product_id (enter here id of the product related for)
2.virtuemart_custom_id (enter here 1)
3.custom_value (product id of the related product).