NOTICE: This question falls into the FAQ page...
What type of questions can't be answered by Vocaloid Wiki
Questions on How to become (a part of the Vocaloid business / an official illustrator / a Voice provider for Vocaloid), and other similar questions.
As previous stated on the Main Page, the Vocaloid and Answer Wiki consist of contributors and fans, thus they have no inside knowledge on how to be a part of the business involving Vocaloid.Contributors are free to give their theories, but take caution of the information provided. See page history
If you were hired by Yamaha or any music program company like Crypton or AH-Software to provide your voice for a Vocaloid, then yes, but not just anybody can provide their voice for a Vocaloid.
They have contests that, in the future, might determine voice providers. NicoNicoDouga is another way to gain thier attention by popularity. So, just know, if you want to be a voice provider, it will be a long road. You don't just ASK Crypton or Yahama or any Vocaloid company as most have a schedule their working to, for example, CFM currently have no plans to release new vocals while their updated and creating english versions of old. Zero-G create vocal specific vocaloids and Internet co rely on professional singers.
UTAU is a free alternative to Vocaloid and you can make your own UTAU with your voice for free.
Just for letting you know, Zero-G Ltd. is holding a contest for an American Vocal. Just sing a song and send it to them.