@Ahmed Reda Chapter 12.2.9 of the ngspice user manual: Filesource (PWL sourced from file). Xschem does not have this very ngspice specific component, but I have used a

vsource.sym

and modified its netlist rule to produce an instance line as explained in the ngspice manual. Attached an example image and the test schematic I used. You can open the test schematic in xschem, copy/paste the voltage source into your circuit and modify the 'filesource' file as required. The file format is not csv, it is a tabular format, one line per time, first column is time, second column is voltage value.

Thanks. @Stefan Schippers