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.
It's never "too late," your chances just diminish the later you are. People get accepted with submission dates all the way up to their application deadlines. There is not one all-encompassing answer to this, so you're going to get very general answers or sporadic answers depending on who you consult.
It won't hurt you (other than financially and potentially being a reapplicant to more schools next year), but it can help you. How much so depends on the programs you apply to and how strict they are with their rolling admissions.
The general consensus is 15 schools is fine. If you are not going to get in with 15 well-selected schools it is unlikely you'll get in with 5 more. That's a pretty fair assumption, but certainly not true in all cases.