Turn Passata Into Tomato Paste?
Passata is crushed tomato.  Tomato paste is a concentrate of tomato produced by cooking for a long time, removing seeds and skin, and cooking further.  They are different products that are going to produce different results, both flavor-wise and in terms of texture.  If I were you, I would not add extra water at all, if you are going to use the Passata.  I am sure it will be good...it will just be different from the intent of the recipe.