You can use the native ability of your machine’s keyboard/fonts… to get some useful characters.
Below screen shows some that were valid (in the eyes of Kicad) and some that were not valid (converted to rectangles and question-marks). I didn’t go through the entire selection, just enough to show here. Thus, you can get ø
EDIT - after more blackcoffee, I went through the full keyboard with the default settings and combinations of Key’s (with Shift and Option keys)… results in Schematic are below…
