Hi fayiqa,
As far as i know if you are designing a WDM system , then i believe you can use WDm analyzer present in the visualizer library of the optisystem. I gives you the SNR/OSNR of each channel in addition to signal power and noise power. However, you can verify it if we are able to calculate SNR of whole deign as a whole. Kindly check the screenshot below. Hope it helps.
Regards