Hello @GCMMMLA, thank you for your post and welcome to the forum 
This topic has been recently discussed here: How to manage (system) fonts? (macOS)
Long story short: we do not recommend adding system fonts into FontBase, doing so could cause some unpredictable effects and unwanted behaviour.
Unfortunately there is no way for the application to manage installed system fonts on either Mac or Windows at the moment.
Hope this helps,
Thank you again,
Yuriy
FontBase Dev Team