There is no real spectral library in SNAP.
But if you want to classify something you can have a look at this thread:
Maybe this brings you already a bit further.
Another options is the spectral unmixing. This result can be further used to classify the image.
The endmembers needed for the spectral unmixing can be defined by setting pins and exporting them by using the Spectrum View. You should give names to the pins like water or forest before exporting them.