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
Its currently impossible for a non-Japanese speaker to get the role of provider for a Japanese Vocaloid in contests and you will not win against true native speakers.
When the studios look for a vocal, they will put up notices in the needed places. For instance, Prima's voice provider came from a opera academy. The studios all have their plans for future releases, thus simply offering might not get you the position. Thus, its quite hard.