How to simulate zener and normal silicon diodes in series to obtain IV curve (with icaps or any other simulation packages)?
I'm not sure about ICAPS exactly, but since it's based on SPICE I think we can safely assume it does a DC Sweep.
For the Zener you would typically just use a zener model. If you have a particular part in mind check on the manufacturers website for SPICE models (example here) or look in the datasheet (often there is a .model listing)
This is how I would do it in LTSpice (I just picked the 4.7V Zener and standard diodes at random from the LTSpice library):  





Here is the Netlist:


  V1 V+ 0 15 Rser=0
  D1 0 N002 1N750
  D2 N001 N002 1N4148
  D3 V+ N001 1N4148
  .model D D
  .lib C:\Program Files\LTC\LTspiceIV\lib\cmp\standard.dio
  .dc V1 1m 10000m 1m
  .backanno
  .end