Hi Saubhagya,
With OPTT config. you can replace 'P' with 'N' and that will ultimately charge to the account assigned WBS of the respective network. There is no point of having a WBS element field in place instead of P or N when the purpose of charging WBS can be filled with 'N' itself.
Even if you insist then in OME9 you can make the 'Project' field as Mandatory entry but that will be in display only.
OR
You may go with a Z-document type and Z-account assignment category configured under MM and make the WBS element field as madatory with the above given selection and assign the same in OPTT, OME9 also.
Regards
Saurabh