The Source object has (double) interarrivalTime as a dynamic parameter, so you can change it dynamicly, at some given event times, setting its value to exponential(1/lambda(t)), where lambda (Poisson parameter) is a function of t = time(), the simulation time.

There are other ways to model this: 1. Let Source object work in "Rate" mode, set rate to, say, parameter R, and change R when you need. The Source will re-schedule the next arrival when the rate changes. 2. Use "Rate table" mode: the table should be of the form: <time moment,rate from this time moment>