Calling a function module in a File to IDOC Scenario

Hi experts,

I'm developing a File-->XI-->IDOC scenario in which I have to populate some IDOC fields calling a custom FunctionModule that resides in ECC.

The input parameters of FM call are fields of the file (i.e. the supplier ID), output value will be put in IDOC field.

Which is the best to call a function module and integrate it in my scenario? I'm new to that.

Thanks!!

Fabio