Please, have a look in E06 example, which includes a stackPopper test.
To run the example with user defined tracks during event processing:
.x load_g4.C
.x test_E06_2.C("g4Config1.C", kFALSE)
See Ex06MCApplication::GenerateFeedback() and Ex06MCApplication::Stepping() functions where adding tracks to the stack is implemented.