I "solved" my problem by putting in the didFinishLaunchingWithOptions of my AppDelegate.m the following code:
NSDictionary *dictionnary = [[NSDictionary alloc] initWithObjectsAndKeys:@"YOUR CUSTOM USER-AGENT", @"UserAgent", nil];
[[NSUserDefaults standardUserDefaults] registerDefaults:dictionnary];
The thing is that i cannot append the text to the user agent itself, but i can write the data i need to differentiate between the access to the web page from the different apps or web browsers.
Thank you all for your help.