Change voicemail message to audio file
To answer the first question regarding changing your voicemail greeting, you're carrier network may have a different number to access your voicemails. By calling the voicemail number, you should be able to change this to what you want.

As for your second question, a solution could be play the audio file whilst your changing it from calling the voicemail number itself.

Again, depending on what carrier you use, they may set different time limits for voicemail greetings. For example Virgin Mobile from where I come from has a set voicemail greeting length of 3 minutes. Not sure what you're carrier has set a limit for voicemail greetings.