On my virtual airline, we have added an airport with ICAO code AAAA named "For the Unscheduled Flights". Also, we have added a schedule with flight number GRW0000, departure airport: AAAA and arrival airport AAAA. The pilots can go to the File a new pirep page and fill this form using the following:
Select Airline: GRW
Flight Number: 0000
Departure Airport: Select the airport he departed from. If he can't find it on the drop down list, he can select the AAAA.
Arrival Airport: Select the airport he arrived to. If he can't find it on the drop down list, he can select the AAAA.
all the other, the same as the scheduled pireps.
If he has selected the AAAA in any of the departure or arrival airport, he has to include the correct icao codes on the other information field. (For example DEP/LGAV ARR/LGTS)
Of course, there is a module which can do what you want by simpilot.