The original engine of the game is kirikiri2.
The result script is going to be using the engine Nscripter.
In kirikiri2, you can add subprogram as you like. On the contrary, Nscripter only allows to use the build-in instructions
I made a mistake. Actually you CAN add subroutine into Nscript and with the help of "defsub" instruction, it can be used as a subprogram.
I managed to wrote a simple script displaying a picture of my classmate and some selections.