stringByAddingPercentEncodingWithAllowedCharacters:を使います。
NSString *originalString = @"単語";
NSString *encodeString = [originalString stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet alphanumericCharacterSet]];
NSLog(@"originalString = %@", originalString); // word = 単語
NSLog(@"encodeString = %@", encodeString); // escapedWord = %E5%8D%98%E8%AA%9E