Hi ,
I think you could use (in the case you are not using CAE)
the following code to define the amplitude:
AMPLITUTE ,NAME=my_amplitude_name, DEFINITION=PERIODIC
1,f,t0,A0
0,A1
where f=frequency, A0=value of free term in Fourier series, 0= coeficient of the first cosine term , A1=coeficient of the first sine term, t0=the initial time
You could see the Keyword Manual for other details.
Then you have to attach the amplitude definition to the load definition by setting the AMPLITUDE=my_amplitude_name
in the CLOAD or DLOAD definition.