This really isn't possible. The Pallof press requires that the loading be a lateral force applied at the hands, resisted from a standing or kneeling position.
To achieve this with a dumbbell, you'd need to either fix your feet to a wall so that your body is sticking out horizontally, and oriented with one side of your body toward the ground. Or you could could have something tightly clamped around your hips in the same position to hold you up. Either way, pretty much impossible, and you'd also have to contend with holding up the weight of your body in that case.
You can easily still train your trunk rotators using weighted Russian twists or straight leg pendulums. Or if you desperately want a neutrally positioned isometric exercise (i.e. "anti-rotation" rather than rotation), you could just lie on your side with shoulders and hips at 90° (so that your arms and legs both poke out in the same direction) and then try to raise your feet just slightly off the ground and hold that position. But then there's still that question of whether Pallof press or other "anti-rotation" exercises are even useful in the first place.