Can I express the numbers at more than one decimal point using 成?
Well, you could say 五成五 for 0.55 or 55%. 99% could be 九成九. 

For example, 九成九的人会参加这会议, meaning 99% of people will attend the meeting. 

Usually, people won't put that accurate. They may just say 大约五成, meaning 50% or so. But 几成几 is a normal expression.