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Nobody knows. I'm guessing you'd have to be like some kind of unknown voice actor singing person, because a lot of the voice providers of vocaloids are from people like that. Also, I believe it is possible before 18, because Yuki's and Oliver's voice providers are under 18.

I gave some suggestions to you. You can be a web singer and upload the songs that you sing or cover on the youtube (e.g. Alex Goot). When you became a famous web singers , they may consider whether your voice was suitable for their setting of Vocaloid. If you were really wanted to be a voice providers of vocaloids and you knew how to speak Japanese language, you can upload your songs or covers on NicoNico Douga and you may got a big opportunities to be a voice providers of vocaloids.

There is also the chance that companies may put up a contest to provide a voice for their vocaloid. This was done with Vocaloid Aoki Lapis and Merli, and it was available for overseas contestants.

But otherwise there is no possible way at the moment to make yourself a Vocaloid on your own.