Is there a simple way to quantize an input waveform in LTspice?
You can accomplish this with a behavioral source and the "floor()" function. Here is an example:

.param vrange = 1
.param nlevels = 16

B1 VQUANT 0 V={floor(V(VIN)/vrange*nlevels)/nlevels*vrange}
V1 VIN 0 SINE(0.5 0.5 100)

.trans 10m