Can you update and add products by the same CSV file ?

[PrestaNow]

Hello everyone , 

I'm new to Prestashop, so  my question may seem weird. 

So I learnt how to add prodcuts with CSV files and how to update them. 

Just to check I update the products by Forcing ID Checks and User Reference as Primary Key.

Let's say I have an existing Database and a CSV files containing new products and some old ones (Which I don't know their IDs in the database). How to add and update products by using the same CSV File ? If I disable using reference key and forcing ID checks, I m going to have duplicates with different prices. 

[Rhobur]

If you have access to the backend, which you must have in order to import a CSV simply go to Catalog->Products, get the product ids you need to import and use them in CSV product id column.

[PrestaNow]

Thank you kindly ! I will make two csv files then !