Hello TingYinHuang,
Welcome to Communities! Some useful info on participating here:
https://communities.vmware.com/docs/DOC-12286
"In this case , Can I add these space to the existing 20TB Datastore and grow it to about 50TB?"
Yes of course - the vsanDatastore will be able to use the space as soon as the Disk-Groups are created.
"or should I creater another vSAN Datastore about 30 TB with these new disk?"
"Are these 2 way supported?"
No - only one vsanDatastore per cluster is possible.
"but this time i will use different Disk Capacity, maybe with 1x1.2TB SSD for cache , 7x 2TB SAS Disk for Capacity, for each esxi"
Just an FYI - vSAN won't really utilise any extra space past 600GB on cache-tier devices (other than for increasing lifespan e.g. replacing dead blocks) so there isn't much benefit to using cache-tier devices larger than ~800GB. If you want to keep the cache:capacity ratio similar to what it is then take this into account and considering adding 2x smaller capacity Disk-Groups (e.g. 2x(600GB cache-tier + 3-4 2TB capacity-tier)).
Bob