You can enter the system using the ZeroPoleGain form:
zpk := ZeroPoleGain(,[1,2],1):
Alas, when converting to other forms the zero/pole will get cancelled. Note that the cancellation option is normally false; it is used to actively cancel pole/zero pairs that are within a given distance, set by the relativeerror option. So that won't help you. What you can try, as a workaround, is to enter an offset as a symbolic parameter and give it a default of 0:
zpk := ZeroPoleGain([1+delta],[1,2],1, parameters=[delta=0]):
The parameter values are used with certain operations such as when generating plots, etc. Here is the State Space representation.
ss := StateSpace(zpk):
use ss in a,b,c,d end use;
Matrix(2, 2, [[0, 1], [-2, 3]]), Matrix(2, 1, [, ]), Matrix(1, 2, [[-1 - delta, 1]]), Matrix(1, 1, [])