Hi Namsheed,
As far I understand your requirement, you can better start from the transaction TRIP where you can enter the advances and , so the request will be created with status "Request Recorded" and after completing the payment for advances or when the employee comes back from the trip, the expenses can be created from PR05 (not as a new expense report. You can select the trip and click on create, so that it will prompt for the schema).
SPRO -> FI Travel Management--> Travel Expenses --> Dialog and Travel Expenses Control --> Define Schema and Individual Field control
Regards'
Gautam