Do you want to simulate the twin lead transmission line in HFSS? Yes, you can do it by simply drawing two conductor and setting appropriate boundary conditions, source and load. The twin lead impedance is related to the radius of the coductor, the space betweent the two metal lines, and the dielectric surrounding the lines, so you can change these parameters to change the twin lead impedance.
Fan has rightly explained the factors. I would like to add the uniformity of dielectric and thickness uniformity of dielectric (distance between twin leads)