Yes it is possible, ive done it a few times
1. the .wav file need to be in the right format, u-Law, Mono, 8KHz
2. the initial voicemail box setup must have been completed (you know when you call the voicemail for the first time and it gets you to setup pin, name, greeting etc, this all needs to be completed first, just do a dummy greeting until you load your .wav file)