Vijesh,
Hmm. I have never heard anyone refer to a Quotation as a 'Pro Forma Invoice'. I guess you learn something new every day around here.
Including quotations within MRP planning could make sense, although I would say most companies are not willing to build product based solely on a quotation.
This is not really a PP issue. Speak to your SD functional colleagues, and tell them you would like for them to re-configure quotations so as to pass requirements to MRP. There is more than one way to approach this issue, and I am sure they will want to interview the client to ask him the appropriate business questions before settling on the solution.
Best Regards & Good Luck,
DB49