AFAIK, no mag extensions. If there were, it'd hardly be an easy DIY (doesn't screw in like a shotgun tube mag). It'd also be rather fragile and awkward on a '95G where even one additional round would have it extending past the muzzle.
Marlin makes a "cowboy' version of the '95 with a long octagon barrel and FL mag tube if you want more capacity, but it isn't near as handy in the woods. Of course, you could have one custom-made and fitted, if you're so inclined.
BTW, it might be good to mention that nearly all LA .45/70s have OAL limitations for reliable feeding. Most are pretty "short-throated", too. This can limit one's viable bullet options to about 450 grs.
Many, if not most, of the "big stuff" is geared for use in reproduction or modern SS actions where the chamber throat is generally longer and the OAL issues are pretty much moot.
While the modern '95 actions and repro 1886 Winchesters can handle pressures which considerably exceed what most repro Sharps, Rolling Blocks, and of course Trapdoors can safely handle, they're still a largish step under the Ruger No.1, modern Hi-Wall, or converted Siamese Mauser. If you need or want performance that's rubbing up against the .458 Winchester, stick with one of these.