I'm not saying that there can't be artistry in the darkroom. I'm saying that in this particular case such artistry would likely not be appreciated. And since you don't know what is on the film in the first place it is very hard to manipulate it in such a way. That requires a lot of pre-existing knowledge about what is on the film.
I spent a ton of time in the darkroom of my dad when I was a kid and it's all lots of fun but in the end what wasn't in front of the camera lens is not going to magically appear on the developed film or the prints. At best you'll end up with a derived work, at worst you're destroying a bit of history.
IANAL but the word you're looking for is "derivative works". In order to develop film, you need to take someone else's work and apply your original interpretation to it.
This is definitely the most stretched interpretation of the term 'derivative work' that I've ever encountered.
Developing a negative does not create 'a derivative work', since developing film is not 'interpreting' it in an original way, it for the most part is chemistry. Do it the same way twice you get the same result.
You can prove that to yourself by cutting a roll of film horizontally in half and then developing the two halves independently using the same recipe. If you do it all by the book they'll come out in such a way that the two film halves can be reconnected to give you the whole images.
I spent a ton of time in the darkroom of my dad when I was a kid and it's all lots of fun but in the end what wasn't in front of the camera lens is not going to magically appear on the developed film or the prints. At best you'll end up with a derived work, at worst you're destroying a bit of history.