Hi Alaa ismail,
if you want to maintained your OM Objects(Departments, Positions, Jobs) number ranges with your existing company number ranges then then do set number ranges in sap as external number ranges.
SAP Easy Access Menu:
1) Use Transaction Code = SPRO
Go To Personnel Management>Organizational Management>Basic Settings>Maintain Number Ranges>Maintain Number ranges
2) Use Transaction Code = OONR to access direct this table
this table carry number ranges just select EX check box for external settings then use to create your OM objects via (PPOME, PO10, PO13, PO03)
It covers your requirement.
Regards;
Bashir