But even if you have the skills for, I don't know if it could works... If you set the game to allow modding, I'm not sure if writing into this .rpf file would be allowed by the game and don't cause a bug or crash the game !
I think using Independence FM is perhaps possible and here how I see it :
In your asi script made in C++ :
> you create a menu which get displayed to the player when he selects the Independence FM
- this menu allows you to set your playlist/songs/internet radio
- to enable/disable Spotify (this mod in a way)
> when the radio is selected and Spotify is on and settings are good you can
- get stream of Spotify (with Spotify API)
- output this stream into an mp3 file which would be located in the directory "User Music". (Then Independence FM should play this mp3 by it self ^^) Bonus
, to be sure the radio station won't switch to another sound (like radio presenter speech sound) :
> If a native function allow to force only mp3 in "User Music" directory to be played then you could implement this too...
If you accept this way to do, I suggest you to start by understanding how Spotify API works (if you don't no yet) because if you can't do what you want to do with your not going anywhere for this mod.
Then when you do so, you have to be able to ouput this stream into a mp3 file. When you know how works those two things and you're able to "play" with them without problems, then you can made your mod ^^ Because the menu is easier to made than the Spotify part
Before anything, I'm going to stream an internet radio into a mp3 file in User Music (with VLC), at least to check if the radio station in the game will play the streamed mp3 file without problem, because if don't, everything above is good to be put in the trash...