NO. Never use ground for a speaker return lead. If your stereo can handle additional speakers, you would connect them to the same pairs of wires used for the existing ones.
You may find that your factory stereo won't handle additional speakers without becomming distorted. The best solution is to use an external amplifier for the new speakers and tap into the existing speaker wires for the amp's inputs. That'd also give you the ability to adjust the balance front-rear.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's