You can't. To buy below 1 satoshi on yobit, you have to wait for them to be listed on yobit first. And even after that, I don't thinknyoubcan buy below 1sat. Go to hitbtc or cryptopia. You have to buy them at 1sat at cryptopia if I'm not mistaken, because basically all you can do is buy what's being offered, which is 1 sat. But on hitbtc, it seems they already create a system to buy and sell with partial satoshi (what is the official terms? Centisatoshi to milisatoshi?)