I suppose technically you
could; Objective-C is a "strict superset" of C, meaning that any C you write will compile quite happily.
However, by doing so you're basically shooting yourself in the foot. For the most part it's a very simple addition, and if you know C already then you'll have no trouble. Also, in order to have access to the SDK libraries (i.e. to actually
do anything) you're going to need to use Objective-C.
Here's a very nice (and short) primer on Objective-C:
http://www.cocoadevcentral.com/d/learn_objectivec/