iOS: POST where JSON is just one argument of many - ios

Question

I have successfully used JSON as arguments to a POST webservices in my apps by making the data representation of the JSON the HTTP body:
//Prep
NSData *requestData = [NSJSONSerialization dataWithJSONObject:<my JSON> options:0 error:nil];
NSString *apiString = <the webservice>;
//POST
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:apiString]];
[request setHTTPMethod:#"POST"];
[request setValue:#"application/json" forHTTPHeaderField:#"Accept"];
[request setValue:#"application/json" forHTTPHeaderField:#"Content-Type"];
[request setValue:[NSString stringWithFormat:#"%d", [requestData length]] forHTTPHeaderField:#"Content-Length"];
[request setHTTPBody:requestData];
//Go
NSURLResponse *response = nil;
queryResultData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
Which works great.
The trouble is, this works for a webservice which is expecting a single JSON argument as the http body.
How can I include JSON as one of many arguments, the way you can use the data encoding of #"arg1=val1&arg2=val2".
For example: image you have a webservice expecting something like
http://mysite.com/api/v1/route/test?data={json:%22true%22}&other=abcdefg
Which must be submitted as a POST.
This, interestingly, has a JSON dictionary as only one argument of many, and not in the body at that. I can't figure out how to add these arguments to the post.
EDIT
Here's the same concept in php. How do you do this in Cocoa?
$data = array('x' => 1, 'name => 'fred');
$json = json_encode($data);
curl_setopt($curl_handle, CURLOPT_POSTFIELDS, array('json_data' => $json));
-- or --
curl_setopt($curl_handle, CURLOPT_POSTFIELDS, 'something=whatever&json_data='.$json);