  Feebly Interacting Particles are a commonly considered extension to the
Standard Model of Particle Physics. In many theoretical frameworks these
particles can explain observed physical phenomena which are in tension with the
current model. \textsc{ALPiNIST} is a simplified Monte Carlo framework aimed at
evaluating past, present, and future, short and long baseline experiments for
their sensitivities to different models of Axion-Like Particles. We present the
extension of this framework to accommodate new classes of Feebly Interacting
Particles with emphasis on Heavy Neutral Leptons. This extension is especially
well motivated, solving multiple of the standing issues with the Standard Model
at the same time. The fundamental importance of inputs on the resulting
parameter sensitivity, and thus the need for a unified simulation set-up, is
highlighted.
