As others have already pointed out, to store primitive C types such as a in an Obj-C object such as an instance of NSMutableArray, you would need to wrap them in NSValue objects first.
As an alternative to doing this - if you are wanting to work with pure C strings in Obj-C, don't forget that you can freely mix C with Objective-C source code, so using a normal C array is a perfectly legitimate solution too.
By wrapping the values into an obj-c array you gain the bounds checking and mutability, but if you keep unwrapping the values to work on them as a C string, you might be better sticking with a plain old C string to begin with, to save the overhead.
If you then want to make an NSString, you can simply use the NSString convenience method stringWithFormat:, like so:
char str[50];
// read characters into the buffer from a file...
// When done, convert to an NSString:
NSString *string = [NSString stringWithFormat:@"%s", str];