Since responses of linear time-invariant (LTI) systems have the same shape for various amplitudes of a given input signal and are just scaled linearly with the magnitude of the input signal, you can just multiply the system transfer function with the magnitue, a, of the step input to find the step response for a*u(t):
G = tf(1, [1 .5 1]);
scale = 1.5;
step(scale*G) % Step response for 1.5*u(t)
title('Step Response for 1.5*u(t)')
The ramp response is the same as the step response of the original system augmented by 1/s. In other words, the ramp response is the integral of the step response and 1/s corresponds to integration in s-domain:
s = tf('s');
step(scale*G/s, 25)
title('Ramp Response for 1.5*r(t)')
