¿Cómo agregar datos en plist? [duplicar]

I have a .plist file. When I write data to plist it does't append but overwrites the previous content.

Aquí está mi código

// get paths from root direcory
NSArray *paths = NSSearchPathForDirectoriesInDomains (NSDocumentDirectory, NSUserDomainMask, YES);
// get documents path
NSString *documentsPath = [paths objectAtIndex:0];
// get the path to our Data/plist file
NSString *plistPath = [documentsPath stringByAppendingPathComponent:@"HistoryList.plist"];

// set the variables to the values in the text fields
NSString *str=[NSString stringWithFormat:@" %@  %@ %@",strname.text,strnum.text,selectfield.text];
self.historyArray = str;

// create dictionary with values in UITextFields
NSDictionary *plistDict = [NSDictionary dictionaryWithObjects: [NSArray arrayWithObjects: historyArray, nil] forKeys:[NSArray arrayWithObjects: @"Name",  nil]];

NSString *error = nil;
// create NSData from dictionary
NSData *plistData = [NSPropertyListSerialization dataFromPropertyList:plistDict format:NSPropertyListXMLFormat_v1_0 errorDescription:&error];

// check is plistData exists
if(plistData)
{
    // write plistData to our Data.plist file
    [plistData writeToFile:plistPath atomically:YES];
}
else
{
    NSLog(@"Error in saveData: %@", error);

}

How to write data in plist such that it gets appended to previous and does not overwrite?