No, not too too late, but it is a kind of panicky move. Most don't have a 9/15 deadline, but you should check the MSAR or individual school sites to get their timing.
I say it's a panicky move because it tells me you haven't had any interviews, or worried "did I do enough?" I think we're all feeling that way, especially since schools don't even start to accept students until 10/15. I predict, even more applicants may add more schools then.
What you might want to do instead is really research and evaluate those 15 schools. Did you apply to your IS? Do you have a clear mission and does that align well with the schools? You see, arbituarily adding more schools is a terrible idea. If instead, you can really express the reasons you want to add one or two others with good, clear rationale, then go for it.