I wouldn't say its impossible, you can do it using pen tool in photoshop or the curve tool on paint. However, once I got a tablet I haven't even thought about thinking about not using it again ^^ If and only if you are serious about your digital art I would strongly recommend a tablet. They are easier and faster to use, allow the use of pen pressure and really there's only so much you can do with a mouse I don't even own a mouse anymore, just use the tablet for everything ^^
It only works best with a tablet, you need the pen pressure to yet the lines edges crisp. ^^ If you look at SAI just above where the layers are you will see the button you click to get a new layer. Next to it is a similar looking button but this one has a pen over it. Click this and a "Lineart Layer" will be made. When on a lineart layer the tools you can use change, the curve and line tools are only usable in these layers.
Ooh, I found it! I had my screen minimized. It wasnt showing it all Just a question, do you use a tablet? And, wow thank you for the quick reply. Could you possibly make a shading tutorial? Sorry for all the questions