Hi,
I do not fully understand your question but I am assuming that you want to read the Infopack source .csv file of a flat file based datasource.
For this you can use following tables in transformation routine (no need of expert routine) to extract the csv file name:
RSREQDONE - Give you the request no. being loaded and source Infopackage id. Fields RNR and LOGDPID
RSLDPSEL - Provide you the csv file name of the corresponding infopackage and then you can get the file name. Field LOGDPID and FILENAME are used for this.
The pre-requisite I can see for this procedure is that you have to know the source Request No. being loaded or the Infopackage name (In this case you don'e need RSREQDONE also).
Let me know if your requirement differs from above.
Thanks
Amit