I'm no expert, but since no one has answered, let me give it a shot.
I don't believe the castor is directly editable.
Different suspension types do it differently, but if you look in suspensions.ini for a car which uses DWB (Double WishBone) suspension, you'll find (example):
WBCAR_TOP_FRONT=0.4654, 0.1025, 0.045
WBCAR_TOP_REAR=0.3935, 0.0925, -0.21
WBCAR_BOTTOM_FRONT=0.5363, -0.1025, 0.06
WBCAR_BOTTOM_REAR=0.4191, -0.0988, -0.2253
WBTYRE_TOP=0.1229, 0.1425, -0.005
WBTYRE_BOTTOM=0.0753, -0.0825, 0.035
Seems like these values describe the 4x DWB suspension mounting points on the chassis (WBCAR_xxx), and the 2x wheel upright top and bottom (WBTYRE_xxx). The difference between the top and bottom WBTYRE_xxx in the y-axis (front-back) would produce the castor angle. I'm not sure which point they are all in reference to, but for your purpose that's not important. Just remember that in AC the y-axis is positive at the back and negative at the front. So, assuming (!) X,Y,Z then making second value in the WBTYRE_TOP thruple larger (more positive) would create more castor. Smaller (or more negative) would reduce castor. The numbers are in meters judging from other suspension values, so small changes, yes?
Hope that give you a lead to work from. Please make backups.
Daggnabit. Ninja'd by
@Beezer215 (and I might be wrong about Y/second)