VerilogA has been supported in spectre for 10-15 years (I forget when) and is certainly supported in IC5X.
You do not attach it to a vpwl or any other source. The code itself is a new source.
To use it, do File->New->Cellview (in the CIW). Set the Tool to "VerilogA-Editor" and fill in the library name as whatever you want, and call the cell name "ladder" (you could call it something different if you like). OK this form.
Then in the resulting editor, paste in the behavioural model code I provided. Change the module name from ladder if you called the cell something other than ladder. Save and exit the editor. It should syntax check the file, and then prompt you to create a symbol. You can then place this component in your testbench, and it will generate the ladder waveform (set vmax=-2.04 when you place it).
I just noticed that my code omitted a line specifying the direction of the pins. Here's the corrected code:
module ladder (outp,outm);
// extent of the signal output
parameter real vmin=0;
parameter real vmax=2.56;
// number of steps in the "ladder"
parameter integer maxcount=256;
// is it periodic? Set to 0 if you want it to be one shot
parameter integer periodic=1;
// period of time between steps in the ladder
parameter real period=10n;
// rise, fall, and delay of each step
parameter real tr=0.1n;
parameter real tf=0.1n;
parameter real td=0;
// internal variables
V(outp,outm) <+ transition(count*scale,td,tr,tf);