how to add object to NSMutableArray

I am trying to read an input file (if it exists) and then want to add a string to that input. My code looks as follows.

NSMutableArray *listData = [[NSMutableArray alloc] initWithContentsOfFile:*filepath*];
// listData = null if the input file does not exist.
NSString *jobName = [NSString stringWithFormat:@"My New Job"];
[listData addObject:jobName];

if the input exists then after addObject:jobName, the listData is updated but if the input file does not exist the listData still gives null after addObject:jobName. My input file (if exists) looks something like.

<array>
      <string>My Job 1</string>
      <string>My Job 2</string>
      <string>My Job 3</string>
</array>

I want to add the string in the existing array of strings or want to create a new array of string jobName if it is not already there. Can somebody help me out. Which method I should use to create a new array of string when the input file does not exist.