Maybe I misread you. At times, the NT authors do a "mash up" of prophets when they quote. Kinda like a hip hop mix of songs. Are you saying that James includes himself in with the OT prophets here (he doesn't mention Amos by name) in a "mash up". It would help me understand what you are saying.
Why --- just because he quotes him. People quote the prophets all the time --- but that doesn't make them prophets.
However, Luke and the Holy Spirit obviously felt that James' words were important enough to be included in the Book of Acts even where they differ in places from those of Amos.