How to add a new character (q + tilde) to the character viewer

If you add the ABC Extended keyboard to your Input Source list, you can make q̃ via q then option shift n. If it doesn't look right in your word processor, you may have to change the font.

You can't add it to character viewer because it is a combination of two characters and doesn't exist as a single unit.

Tom Gewecke points out the simplest approach to entering a combining tilde onto a letter q.

The character viewer has a category of combining characters and there is a tilde combining character. You can see all of these by entering combining, or just combining tilde in the character viewer search field. Once you find it, I would add it to the character viewer favorites. Then enter 'q' immediately followed by double-clicking the combining tilde character to get q̃.

You can type this using the "Unicode Hex Input" keyboard layout,

by holding down the Option key and typing the Unicode values:

q̃

obtained with (Option key down)00710303

Then you can copy this and paste. I did not find a way to add this to favorites.

Another easy alternative is to add this as a text expansion in System Settings->Keyboard->Text Replacements

so for example you can type two letters of your choice (say qt - for q tilde) and have it be replaced by q̃.