Hi Faizan,
You can achieve this in Standard SAP. Please follow below mentioned steps.
1. As mentioned in earlier reply, set the default Person Resp. at PD level.
2. Write a substitution rule at WBS level that copies Person Resp. from PD. (see below screenshot)
3. Setup the field control for WBS where you will set the Person Resp. field Display-only in T-code OPUK. (see below screenshot)
P.S.: Here Project Manager is Person Resp.
Best regards
Aditya