There are several ways to generate new glyphs
Press ⌘+⇧+G
In the up comming dialog, enter the glyph names for the glyphs to generate and press
.Select the Font View
In the sidebar, select the language group (e.g. Latin > Wester European) you want to add glyphs to. You see the missing glyphs count in the badge.
Right click the row.
In the popup window, select all glyphs you want to generate and hit
.