@Nirmal_Kumar you aren’t able to use the CSV Importer to add any information (subtasks, custom fields, dependencies, etc) to an existing task. @Bastien_Siebman the Great is right, of course. The CSV Importer is specifically designed to only introduce new information into your project.
That said, there are workarounds but they require some manual effort. You may already know of the advanced action within the Task Details to “Make Subtask of”. The option that is above the one highlighted in the image below:
You can use this action to easily nest any new tasks you import as subtasks of another task. The easiest way to find the task you want to make a task a subtask of, is to paste the parent task’s link address into the search field when performing this action. When populating your import file, you could also use a custom field to note what parent task you want the task to be a subtask of. This might help expedite the process of making the new tasks subtasks of another.
There isn’t a bulk “make subtask of” but if that is a suitable solution you might consider upvoting this post: Make it possible to move multiple tasks as subtasks
You might also upvote this feedback as well: Import to update existing tasks
Hope this helps! 