If your manager/NAS is providing SNMP access, you can try to certify it as a device in PM. As per SPECTRUM, you can certify there also, but it will be more oriented to providing status and trap mapping.
Another option would be to create an extension for SysEDGE that would present data from NAS manager in SNMP, so you would then have to certify the SysEDGE agent with custom branch in both PM/SPECTRUM. It's not so straight forward, it's time consuming, but it will work for sure. The important thing here is how much information you're trying to get from the NAS/manager and if there are any ways of talking with the NAS/manager using any type of API available (CLI, REST, so on).
You can try to have a look at the EMC probe in UIM to see how it's built and how is reading the information from the NAS/manager and use that in your custom extension for SysEDGE.