You do not need to use Quartus's text editor to edit HDL code. The source code files are in your project directory and they are normal ascii text just like a .txt file. If you other editor doesn't generate any funny characters or save in a binary .doc, .docx, .odt, .rtf, then you can use it directly on the files. And then you just need to compile in Quartus.
There exist other HDL ascii text editors which will also properly colorize the mnemonic. Google Verilog VHDL text editors.
You do not need to exit Quartus when externally editing. After you make a 'save', Quartus will automatically detect the change when you re-compile your project.