How to add newly discovered cell node as member of exadata storage server grid in oem12c?
Got answer for my question.Here is the command to add newly discovered cell nodes under existing exadata server grid

emcli modify_system -name='Exadata Grid da13.myhost.com' -type='oracle_exadata_grid' -add_members='da13cel04.myhost.com:oracle_exadata'

output:
System "Exadata Grid da13.myhost.com:oracle_exadata_grid" modified successfully