You asked: "Was Cain's curse an addition to what already existed?" I think the answer is: Yes.
Now I'll explain my answer.
God created man from the dust of the ground. Genesis first describes the creation of the heavens and the earth, so the use of the word 'ground' implies it comes from the earth (or the planet that God created). The 'dust of the ground' includes elements and organic materials of the planet earth, therefore when God created man he created another 'earth'.
Just as the planet we live on has soil, so does man. Consider that our planet has a skin with all of its life and water on the outside, and we have a skin with all of our life and water on the inside. Our soil is internal. The use of the word 'soil' as related to people is highlighted throughout Scripture.
A close reading of Genesis shows me that man (Adam) was a child that was given everything and responsible for nothing. The only thing he had to do was name the animals and not 'eat the fruit' of a certain 'tree'. He had no concept of death, and he did whatever he wanted to do. Another way of describing Adam is that he was a spoiled child of great privilege.
It is possible that when Genesis first described Cain and Abel's interaction with God they were young and learning. Cain's life may have mirrored the life and actions of his father, while Able's life probably mirrored the life and actions of his mother. I have not found where Adam gave thanks to God for anything, while Eve did.
I suspect the soil Cain worked was a reference to his treatment of other people. My impression is that he was a harsh and cruel man who bullied and forced his family and others to get or do what he wanted. I feel this is the reason God was not pleased with Cain's fruit. Able, on the other hand, tended flocks. This describes a kind and loving relationship with people and animals.
The curse may have been that Cain carried guilt for his actions and the killing of his brother for the rest of his life.
By faith Abel brought God a better offering than Cain did. By faith he was commended as righteous, when God spoke well of his offerings. And by faith Abel still speaks, even though he is dead.
1 John 3:12
Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother’s were righteous.
As a side note: Again, imho, consider that when the words 'earth' or 'world' are used throughout Scripture, I understand they are directed to the person reading the passage. As an example: John 3:16 "For God so loved the world (you, me) ..."