It’s no different from how you type those characters to other applications, it depends on what tools your operating system gives you. It could be done with certain compose key sequences, for example on my Linux desktop I type μ by Compose, m, u. It could be done with a pop-up application that presents a grid of characters where clicking on a character inserts it into the input buffer. It could be done by copy and paste of such characters from another window. So this is not a feature that KiCad needs to provide.
It’s not just for Greek letters like µ or Ω but accented letters like the è in caractères, which for me I entered as Compose, `, e. You use whatever your operating system provides for entering such characters.
When I say OS, in the case of Linux, it also depends on the desktop. I use the Compose key method as it handles the vast majority of my needs like typing accented characters in other languages, the ° symbol for temperatures and GPS coordinates to Maps, and fractions like ⅔ which are typed as Compose, 2, 3. For less common characters I can bring up kcharselect which gives access to the whole Unicode shebang (is there a hebang? ). For emojis, I can bring up the emoji selector popup. So there are plenty of existing solutions in this space.
Incidentally font is not needed in the title, as characters have an independent existence to their graphical depiction.
You also robbed OP of their second question in your title.
Copy the original post to a new topic, and suppress the first question in that, don’t know if this is possible. Or tell them to repost and not put too many questions in one topic.
I always forget the character combinations, but in Linux Gnome, there is the Characters application that can be easily opened and searched for characters (it remembers the latest used ones). In Windows, there is a similar application, Character Map.
Strangely, the ohm sign has its own character in Unicode, U+2126, but this doesn’t work well in different applications such as KiCAD. As mentioned above, it is better to use the Greek letter Ω, which is U+03A9.
I use KCharSelect on Linux and used the Character accessory on Windows for all special characters. Never bothered to learn the key combinations.
Note that KiCad provides no preview of text in the font you’ve selected (be it the KiCad font or a TTF one if using v7), so you only see if the font supports a given character when you validate the text. A bit tedious for some special characters that are not ultra-common. A text preview with the selected font would be an appreciated addition. I don’t know if it’s planned for v8, or planned at all.
Personally I would try to avoid using these sort of suffixes. Whilst they are generally ok in KiCad, any external scripts may not be so tolerant. Some time back I had a BOM which included resistors with Ω symbols - this caused major confusion and corruption of a Python script that processed it.
My attitude is that one should boycott (or girlcott ) applications that don’t support Unicode in this day and age, but I know this is not always feasible.
). For emojis, I can bring up the emoji selector popup. 
So there are plenty of existing solutions in this space.
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) applications that don’t support Unicode in this day and age, but I know this is not always feasible.