IPhone : Save NSMutableArray without Core Data

I need to save a NSMutableArray to disk in order to store my application data. I know there are a lot of similiar questions out there but none of them I found covers my question.

I do not want to integrate CoreData just for saving one NSMutableArray. Normally I would go for implementing the NSCoding protocol and using the NSKeyedUnarchiver. Unfortunately my data model class has some foreign classes from a library which do not implement the NSCoding protocol.

So which is the best way for me to store my array ?

This is what I tried but because of the given reasons it won't work:

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
#import <ForeignFramework/ForeignFramework.h>

@interface DEModelClass : NSObject <NSCoding>

@property (nonatomic,strong) ForeignFramework *foreignFramework;
@property (nonatomic,strong) UIImage *image;
@property (nonatomic,copy) NSNumber *number1;
@property (nonatomic,copy) NSNumber *number2;

@end


#define kEncodeKeyForeign   @"kEncodeKeyForeign"
#define kEncodeKeyImage      @"kEncodeKeyImage"
#define kEncodeKeyNumber1     @"kEncodeKeyNumber1"
#define kEncodeKeyNumber2     @"kEncodeKeyNumber2"


#pragma mark - NSCoding
- (void)encodeWithCoder:(NSCoder *)aCoder {
    [aCoder encodeObject:self.foreignFramework   forKey:kEncodeKey];
    [aCoder encodeObject:self.image  forKey:kEncodeKeyImage];
    [aCoder encodeObject:self.number1 forKey:kEncodeKeyNumber1];
    [aCoder encodeObject:self.number2 forKey:kEncodeKeyNumber2];

}

- (id)initWithCoder:(NSCoder *)aDecoder {
    if ((self = [super init]))
    {
        self.foreignFramework = [aDecoder decodeObjectForKey:kEncodeKeyForeign];
        self.image    = [aDecoder decodeObjectForKey:kEncodeKeyImage];
        self.number1   = [aDecoder decodeObjectForKey:kEncodeKeyNumber1];
        self.number2   = [aDecoder decodeObjectForKey:kEncodeKeyNumber2];

    }
    return self;
}