/Macintosh HD/Library/Fonts/ is a Mac OS X system font folder. So, the fonts located there will already appear under System Fonts and Local in the left-side navigation pane in Suitcase Fusion 4. Suitcase Fusion 4 can actually disable system fonts, but I prefer to add them from a non-system location with default settings so they are copied to the Vault.
One way you can explore your font configuration is via View > Columns > Location.
If you’re curious about where your Vault is located, the configuration is under Suitcase Fusion 4 > Preferences > Type Core.