If it needs it I would!  I do anyway, makes it much more paladable, but some people feel it's best not to because sodium retains water.  I feel that if it makes the soup taste better to you, then you will stick to the diet, so go for it!  But that's just me!  I'm sure the onion soup mix is loaded with salt, I just use bouillon cubes and they are all salt!  The rules say yo can use any spice and salt and pepper are spices!