It seems that for CAS storage (Atmos and Centera) you must use the CAS setup within ECS. This rules out using HDFS - only ECS buckets that support HDFS can be used, not object/CAS.
Conversely, Azure, S3, Swift, etc. are all HDFS-compatible, and many object storage solutions offer S3 interfaces. One is Caringo, offering HDFS integration + object storage and data retention capabilities on commodity disks.
EMC would like you to replace Centera with ECS, and will even let applications built with the Centera API talk to Centera and Atmos through ECS, but you can't have your HDFS cake and eat it too it seems, the Centera API doesn't fit unfortunately.