If you want to do this often, you can create a keybindings file in your Library to map it to a key combination.
In ~/Library create a directory named KeyBindings. Create a file named DefaultKeyBinding.dict inside the directory. You can add key bindings in this format:
{
"x" = (insertText:, "\U23CF");
"y" = (insertText:, "hi"); /* warning: this will change 'y' to 'hi'! */
}
The LHS is the key combination you'll hit to enter the character. You can use the following characters to indicate command keys:
@ - Command
~ - Option
^ - Control
You'll need to look up the unicode for your character (in this case, ? is \U2234). So to type this character whenever you typed Control-M, you'd use
"^m" = (insertText:, "\U2234");
You can find more information here: http://www.hcs.harvard.edu/~jrus/site/cocoa-text.html