I know it's easier with VPULSE.
However, I would like to create my own VDC with an initial step voltage, stabilized voltage, added noise, etc. to match the actual measurement environment.
Is it possible to make it take multiple inputs and output a voltage in verilog-a?
Verilog, no. Veriloga, yes you could. I have made widgets which
(with some outside-the-box scripts) make complex digital
waveform stimuli from VCD files that present as pulsed (NRZ)
voltage. A bit of fiddling to get it hooked up the way Spectre
netlisting likes to work and "refresh" the content, beware the
"buffering" of verilog.va views and the need to "reset" after any
editing.