Hi,
if you use BAPI_PRODORD_SCHEDULE after BAPI_PRODORD_CREATE you can change the SCHED_TYPE. You have in the customizing (OPU3) a value for schedule type = Backwards.
Therefore you give in BAPI_PRODORD_CREATE any date in BASIC_END_DATE followed by BAPI_PRODORD_SCHEDULE with the correct startdate and schedule type.
Greetings
Achim