Is it possible to install a threaded cartridge bottom bracket in a threadless shell which has a PF30 installed?
If you want to be able to install any ISO threaded BB, the way to do that is use a press-in adapter.
They have the disadvantage of not necessarily solving the creaking issues that might have gotten you to this point in the first place.
If you just want a simple way to run common 24mm or 24mm/22mm cranks, there are adapters that plug in to an existing PF30 or BB30 for that.
If the shell is metal and you want to run square taper, you can in theory get it cut to T47 and run a Phil Wood BB in it with their T47 cups. Despite being insane, this is the solution most likely to result in zero creak issues.
In addition to the press-in adapters, there are also screw-in "conversion" bottom brackets that actually do a great job of addressing the creaking that comes with press-fit bottom brackets.
These work well, and do a great job of eliminating bottom bracket noise, but they are a bit expensive.
One huge advantage is that the bottom bracket screws together and can be installed with a standard Shimano-style bottom bracket tool - no bearing press is needed (although the contact surface for such a tool is really small on BB86/BB96...). The fact that the two sides screw together means the bearings will be properly aligned and are held in place much more securely than press-fit bearings are.
To install one, you probably have to remove your current PF30 cups. Given the consequences of a botched removal, I'd recommend either paying a bike shop to do the removal or (probably cheaper) getting a proper tool, such as the Park BBT30.4. Just make sure you properly brace the frame before trying to knock out the installed bottom bracket.
I've had good results wrapping the threads with teflon plumber's tape and then applying a bit of blue Loctite to the threads when installing. All the creaking stopped and hasn't returned in a few thousand miles total on two bikes.