Maor, can you please have a look at the solution in the docs and see if it's accurate?

At which point exactly the operation fails?
Are you trying to test a Disaster Recovery scenario or simply migrate a Storage Domain from one DC into another DC/Setup?

Sure,
You can take a look at the following wiki:
  https://www.ovirt.org/develop/release-management/features/storage/importstoragedomain/

In this wiki you can find the following flows:

For migrating entities between different Data Centers at the same engine setup you can take a look at the following flow:
  Work flow for detach and attach Storage Domain with entities - UI flow (see [1])

For DR solumtions you can look at the following flows:
  Work flow for Import File Storage Domain - UI flow (see [2])
  Work flow for Import block Storage Domain - UI flow
and also work flow for Import local and gluster Storage Domains

Keep in mind there are few restrictions which should be followed while using DR solution, one of them is that the once a disaster occurs the Host should be rebooted before trying to import the Storage Domain into the new setup to prevent sanlock issues.

Please let me know if there is anything unclear in the wiki so I will fix it ASAP.

[1] https://youtu.be/DLcxDB0MY38
[2] https://youtu.be/YbU-DIwN-Wc

Hi Yoshinori,

From the customer ticket it looks that the user was trying to use a Storage Domain in a Data Center which was probably already existing in the setup at another Data Center.

For using a Storage Domain which was already exists in the setup the user should first detach it from the previous Data Center and then attach it to the new Data Center.

After the Storage Domain is attached the user can import the entities to the Data Center using the following flows described in the doc:

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Virtualization/3.6/html-single/Administration_Guide/index.html#Importing_Virtual_Machines_from_an_Imported_Data_Storage_Domain

and 

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Virtualization/3.6/html-single/Administration_Guide/index.html#Importing_Templates_from_Imported_Data_Storage_Domains

Regarding describing a full testing Disaster Recovery flow:
RHEV provides features for handling Disaster Recovery although describing a full DR solution is something which can be handled in various different scenarios and it is supported by different vendors so it does not described in the doc.

In the documentation of RHEV I haven't seen any mention of the ability of migrating entities between different Data Centers in the same setup.
If this is what was missing for the customer, I will open an RFE for the doc team to add this description in the doc.

Please let me know if this is what should solve this or is there anything else that should be done.

Pushing out to 3.6.6 until needinfo is provided, at which point we'll be able to ascertain the proper target.

Please let me know if there is any additional assistance that you need from us

Thank you,
Maor

Information provided, closing.