1. After several tests, it looks to me that the same schedule can only be used in a policy and not in different policies. That sounds weird to me as for what logical reasons behind that. In some cases, what if I wanted to use the same schedule for different policy? why cannot be allowed? That was my original question.
2. As different than what @hmoubara said, " snapshot policy add-schedule " only can be used if I use a different schedule into the same policy. So, the command as suggested below was incorrect:
cluster::> snapshot policy add-schedule -policy >policy name< -schedule daily2030 -prefix <>