How to change a parameter in Service/Activity other than MainActivity for testing

I want to write a test app that invokes another activity/service of my product code. But I need to make some customization during the creation of the activity/service, that means I need to set a parameter to be test app's parameter in these activity/service during or before the onCreate(). But I don't want to alter product code to add these parameters. Is there anyway I can set these parameters using test app's MainActivity?