It could be done with reasonable results. That is NTP thread you have there and is cone shaped, though it does come in straight also. If you screw the cone shaped threads into plastic and try to tighten it much, it may cause issues.
I'd use 3/8th inch instead of 1/4 inch if possible and if you are lucky you could find a locking nut for the NTP threads. likely at a marine mechanics shop. They use about the same thing with a locking nut for fuel tank nipples.
Rough up the reservoir with sandpaper, clean the threads with solvent and maybe wire brush them some. Rough up the locking nut with a wire brush.
Screw the locking nut onto the fitting as far as practical, smear the threads with a quality epoxy glue (Henkel/Loc Tite makes some good stuff) screw the fitting in by hand (snug not tight) and then lightly tighten the locking nut flush (snug not tight) up to the reservoir, glue should squeeze out of the joint. If you get it clean and oil free it should last and be actually stronger than the plastic was.
A second solution is to screw the fitting in (snug not tight) and then pile your epoxy around the joint. you have to keep scooping the epoxy towards the fitting until it starts to set up and gravity doesn't make it flow, you want to build a ridge around the joint. A fast epoxy can make this much easier. It will likely last and the extra epoxy will add strength, but most epoxy does shrink with age and can crack with temperature changes as it expands and contracts. I've had reasonable results with radiators doing this, most of it is preparation and cleanliness, make sure it is clean and rough it up, epoxy won't stick to oil, water or antifreeze very well.
Some of my radiator repairs with epoxy have lasted 4-5 years. Use a quality product, Permtex, Loc Tite, JB weld (though it isn't as good as it used to be)