If it's not an actual regulator, and just an on/off valve, then you can't run anything inline. You'll need to remove the outlet, as I said, and find a valve rated for 800 psi or better that will screw into a 1/8" NPT port. Except that I doubt you'll find anything that's much more precise than the included valve. So, in your shoes, assuming what you have isn't a real regulator (as in, it doesn't actually reduce the pressure - it only opens the valve a bit at a time), I'd just leave it as is. And upgrade when you get sick of it.