During a major event, the news stations will almost always be there to cover it. If you want to get the video of them covering it, you should arrive to the location unreasonably early. This way, you will be able to wait out their arrival and capture all the footage that you would like.
You may want to try and get some inside information as to when they may show up, but this can be hard to do unless you already have some connections at that affiliate. From here, you can videotape their arrival as well as any interviews they get. There are no restrictions against videotaping a news organization gathering their footage so you should be in the clear.
This footage will show much of the event coverage without having to use their official footage. Try videotaping them consistently so you can also get “off the record” footage and dialogue from the news agency.