You need to contact the Emergency Management/Emergency Services Communications Director for the Counties you want to add that are out of your FPD. He can give you Countywide info, and get you pointed in right directions to talk to each VFD chief to get permission to add their stuff. MississippiPI, as well as myself have been Volee's and did the exact same thing your talking about doing. We contacted each agency we wished to add to get permission to do so. Also, make sure you have State Fire 1, 2, and 3 in the radio, as most multi agency incidents will shift everyone on to one of those for Interop Fireground, unless you are lucky enough to be in an "big" County that has multiple FG's, or is on an TRS or even MSWIN.