How to have multiple sources of emission and implementing a time feature

Hello everyone,
I am using the human phantom (MIRD) and I would like to ask for a way that my source of emission would vary through time, in other words I would like to have my electrons emitted from one organ and then after a set period of time there would be a second organ that’s gonna be another source of emission.
In the human phantom example where do I find the GPS that would allow me to make the changes.
Thank you.