The problem is that for the ball to transition immediately to rolling there must be a horizontal frictional impulse at the ground during the collision. This invalidates your angular momentum equation. To fix this, suppose the impulse at the collision point is J. In terms of that, you can now write the angular momentum equation for the rod about its hinge and that for the ball about its pivot (the point of contact with the ground). J can then be eliminated between the two equations. Indeed, we can generalise the problem to two objects with MoIs I1, I2, about their hinge points, some initial angular velocities about those hinges, and respective final angular velocities. Let the line of action of the collision impulse be r1, r2 respectively from the hinges. (I.e the perpendicular distances to the impulse vector.) It turns out that the usual linear velocity relationship still arises. If, at the point of impact, the first body's velocity goes from u1=ω1r1 to u'1, etc. and KE is conserved then u'2-u'1=u1-u2.