Dear Jorge
Thanks for your question. So far (as of V.1.3) UQLab does not support discrete distributions.
What I mean is that you can indeed define your own custom distribution, which may be used for sampling, but the other modules (e.g. PCE) will not “support” it, i.e. will not work properly. In the case of PCE, the reason is that the way arbitrary polynomials are constructed for custom continuous distributions will not work for discrete ones.
To solve your problem anyway, I suggest you use a continuous uniform distribution, and use a rounding to get the discrete values that are needed as inputs for your MODEL. This can be done after uq_getSample(N), e.g. using the floor or ceil functions.
Best regards
Bruno