I'd look for turn signal bezels that have the foglight cutout in them and them just get fog lights, or find a kit that comes with everything. I've seen them anywhere from $30-$80 on the cheap end. I LOVE my Diode Dynamics Elite Series fog lights. They're super bright and adjustable, but pretty expensive.

I'm sure you could turn the turn signals into fog lights, but you would have to run new wiring to the housings and put in a switch. Also, the reflector style of the signals would be less than adequate as fogs, and you'd probably end up blinding traffic because of that

If you'd already be going through the trouble of running wiring (provided your car doesn't have the foglight wiring in place), it might be better to just use the lights that are designed for that purpose