Hi André.
One option would be to use heuristic SAP_DS_03 to create fixing intervals in the resources.
Additionally, the function "Firm Objects", which can be executed as a Planning Run, can also be used to firm operations of orders (for example, when you specify 'Operations' as the 'Planning Object' in /SAPAPO/CDPSB0).
Coding-wise, when a Planning Run is executed with Firm Object or Unfirm Objects, Function Module /SAPAPO/FDIS_DISPATCH_FUNCTION is called, which then later on calls the Fix/Unfix functions itself and, later on, FM /SAPAPO/EFPL_OPERATIONS_FIX, which fixes the objects.
Regards,
Tiago