Internal Storage expansion

My internet has gone off a couple of times or my cameras have lost connection multiple times and everytime they stop recording, now in many of the latest cameras i can leave batteries in them but have them plugged in so i would hope (i haven't tested) that if the power goes off then the cameras can stay active whilst the batteries are charged but then comes the issue of recording. I would be more than happy to put my own sd card in as a storage space where the camera can save footage when the Internet connection is unavailable which it can then upload when restored. Considering this is a security system this is a mayjor hole in the current model.