Hi Sridhar,
Not really sure if I understood your question correctly, but hope my input will be useful.
Subproject in SAP PS is a kind of grouping label. That means it can be a subproject (different labels within same project) or a super-project (same label cross multiple projects). Basically you can apply a matrix cut across projects.
You can maintain subprojects using transaction OPUM. That is just a code and description. Once you use a subroject field for one WBS within project definition, the system will ask you to maintain it for all WBS in a project.
Subproject has nothing to do with Projects. It is not about linking different projects into any hierarchy.
Regards,
Paulo