Most of the Kicad components are not compatible with Ngspice. eSim team has created their own set of libraries which are labeled under eSim_*(eSim_devices, eSim_subckt).You need to add UJT using Device modeling or subcircuit builder. Tutorials on device modeling are available here: http://spoken-tutorial.org/tutorial-search/?search_foss=eSim&search_language=English and for subcircuit builder, you can refer eSim user manual here: http://esim.fossee.in/downloads.

You can use any one of these methods to add the device to the library. Or the other way is by using the equivalent circuit of UJT with diode and resistors.