If you have an idea of a piece of music you would like to play or a song you would like to sing, speak to your conductor or your ensemble manager.
The more concrete your suggestion, the better, so if you have an arrangement in mind, mention it. The absolutely best thing to do is to source the music yourself (as in say where it can be purchased).
If you have a source (ie where can we procure the arrangement) etc, your wish is more likely to come true!
Singers can submit their suggestions HERE.