Funny enough I was talking to a guy just last week about this, he's a fire safety technician who comes in to test our stuff works.
I asked him the same (is it safe, fitting a reg) and he said that it's not a problem at all, they convert extinguishers all the time for use in welding as over here it is way too expensive to buy a tank so the only real option is to hire one and that can be a hassle or find a cheap alternative.
1: make sure it is a co2 one first (red with black band)
2: make sure it is empty, if need be empty it :-)
3: I will need to take it to a fire extinguisher place to refill it.
They remove the top handle, fit a standard gas fitting, test and cert the tank and fitting and will fill it up for me.
About a half hour job, drop off, go have a coffee, come back, pay, and I'm done.