can we change the item unit price at Purchase Requisitions level ?
You can not change the item unit price on purch requisition lines. You have to do a little of development to change the code.
Actually if you want to enter unit price on purchase requisition, you have to do RFQ process. You have to create an RFQ and enter the RFQ replies coming from the suppliers and approve the reply. So that you can change the item unit price at PR level.