NL-Series nodes are designed for Archive workloads and, at least for now, do not support SSD.  If your Archive workload is one which would benefit from the use of SSD for metadata acceleration (and many would), the best solution is to add S-Series or X-Series nodes with a large amount of SSD, which will then allow the activation of Global Namespace Acceleration, extending metadata acceleration into all nodes pools.  The current guidelines for GNA are that:
Nodes with SSD and NL-SAS drives are X400s.  NL nodes are designed for archive data. 
Can I have SSD drives in NL nodes?