Specify constraints ARMAX model

I want to construct a model like: y(t) = a1y(t-1) + a2y(t-2) + a3y(t-250) + ... , with autoregressive orders only at specific lags, how can i do this versatily without constructing idpoly objects and manipulating 'Value' and 'Free' properties of the object structure? My problem is that doing so i have to set 250+ orders to my model