カテゴリでプロパティを追加する ref: http://qiita.com/items/386dd8e1a294c37085c6

gistfile1.m

#include <objc/runtime.h>

@interface NSObject (AddProperty)
@property (copy) NSString *string;
@end

@implementation NSObject(AddProperty)
- (NSString *)string
{
    id result = objc_getAssociatedObject(self, @"string");
    return [[result copy] autorelease];
}
- (void)setString:(NSString *)string
{
    objc_setAssociatedObject(self, @"string", string, OBJC_ASSOCIATION_COPY);
}
@end