In order to start your new object off as being Base Game Compatible, you can clone a Base Game Object. To do this conifgure the SimPE FileTable. You do this through the Extra menu, choosing Preferences. Then select the FileTable button on the left hand pane. Uncheck all the tick marks except the "Include CEP" and "Include Original" and then press the Reload button and wait. It will take a while - have patience! When it is done, you can then use the Object Workshop and only objects from the Base Game will be shown.
If you have this setting and you are browsing through BHAV code that is based on later EPs you will see "Unknown BHAV" instead of the BHAV name.