I’ll refer you to the fundamental theorem of software engineering:
If you’re dead set on not solving this by literally opening that file and adding that line, then you can solve the problem by, instead of directly calling hello, call it ‘indirectly’ through a different function:
# suppose `that_file.py` contains the `hello` defined in the question
# you can add stuff
# no longer call `hello` directly
But do think about the costs in terms of complexity and the maintenance costs of updating all references to the original hello. But hey, updating all the references is something you only have to do once; then after that, you can make changes to indirect_hello almost for free.