Horn antenna for radar sensor
Yes, what you suggest is possible.

I used to work on a 60 GHz radar solution and there we designed horns and lenses made from plastic, some of which were 3D printed. Both were used to "focus" the radar beam. That radar product is a chip (IC) with build-in antennas so these antennas were quite small. That makes the EM waves behave almost as if they were all coming from one spot.

A challenge with your antennas will be that the antennas are distributed over a larger area. Not all antennas can be in the "focal point" of the lens or horn at the same time.

You might also want to contact InnoSenT and ask them if they have a radar product that has a more focused beam.

A horn antenna generally has a coax or transmission line input and that will have a shield which needs to be connected to ground. So the antenna indeed will require a ground connection.