A family of Microsoft word processing software products for creating web, email, and print documents.
Hello,
As far as I know, the only way to add the font to Office apps is to download the font and install it, where installing the font will show up in the Font book of Mac and once you restart Office apps, the installed font should show up in Office apps. If the Custom Font installed is not displaying in the latest version of Office apps and even after restarting Mac, please refer to the steps in this article: Third-party installed font does not appear in the font list in Word for Mac.
Moreover, if it's a free custom font, you can share the detailed name with us, so we can test at our end and check the result.
Best Regards,
Chitrahaas