batches with different half-life - linear PK

Hello NONMEM Users, Sometimes the same subjects receive the same drug from different batches. Potentially, half-life can be different accross batches. Theoretically, it is easy to model if PKis linear because NONMEM superimposes single-dose processes to obtain predicted concentrations for multiple doses. Does enyone know how to implement it in NONMEM? Are there any classes or publications which can help? Thanks! Pavel