NetApp systems does have some information available via SNMP, but you are correct that you will need to use Custom Pollers to view the information into NPM. Here are some that I was able to find. NetApp Custom Pollers
Just to let you know that Storage Manager is able to connect to NetApp via the SMI-S Provider to report back information about the system including performance monitoring, capacity planning, and reporting. This product may be more beneficial if you have multiple SAN/NAS systems that you need to monitor for volume usage.