Hi Pras and Raymond,
After I investigated the issue, standard inbound process code HRMD does not allow to create any objects with other number ranges. It will always use the number that comes in the IDOC.
Reason is to allow integration since SAP it is not maintaining any links. So, any further changes done to that object in sender side, once it reaches receiver, system won't be able to understand which object should change, or if it has to create a new one again.
Project already had a custom table that links OM objects between both systems. So I ended using the customer exit of FM IDOC_INBOUND_HRMD to first read this table and if a link already exist, then converts object number. If it didn't existed, then used RH_GET_NEXT_NUMBER (if I am not wrong) to check next available number in range, and allow standard process code to create the new one.
Do not know if this is the best approach, but considering everything, at least for now it suites our needs.
Regards,
Martin