Multiple Stepper Motors in One Shield?
Depending on the shield you can connect an external powersource through mosfets or transistors (Depending on the size of the steppermotor).

But if it is smaller motors with a 12v powersource, i really don't se why you would do that, since stepper motor controllers cost about $2-$4 each. Being cost effective i think that writing this question would cost more than actually just go and buy 6-8 controllers. (Don't buy the shield ones, they are overly expensive)