The easy, cheesy way is to put a voltage source in series
with the gate. Your floating gate transistor is never floating
in any useful circuit, the control gate always has R/W ckty
attached and so you simply need to represent the =(Q/C)
delta gate voltage for a functional simulation.
If you want to model read and write dynamics you'd perhaps
prefer to use an initializing current pulse (Q slug) at time=0
and model the tunneling actors behaviorally or by primitives
(e.g. a trivial zener(s) and a huge resistor).