With the newer NPM versions there is an even better way, by selecting Manage Pollers from your settings you can create a CPU and Memory poller that will replace the current Solarwinds CPU and Memory if the MIB is found when you List Resources for the node
You can make the poller here and define the datasources (OID's) and even do the math right here:
Once done anytime you scan for nodes or list resources for a node that has these MIB's it will give you the below option
This way you don't have to worry even about custom pollers and this is picked up by default CPU and Memory alerts.