Gomathi,
APO cannot create network orders / collective orders, it can only read them from ECC. My approach to tackle this was to plan for the leading / parent part# via SNP, convert to PPDS, publish the planned orders to ECC, perform an MRP run which creates planned orders under direct production for the sub-assemblies. At this point you can run a report to convert Planned Order to Production Order in ECC which creates collective orders.
In APO, you need to run the SAP_PP_MRPDS heuristic to read order network from ECC to maintain appropriate pegging relationship.
Regards
AK