Dynamic Excel

Hello all. I'm new to Pentaho.

I'm working on a transformation that needs to read an Excel file as an input, take a column from it and count the rows it has.

The problem is that the Excel file and the column for the count cannot be hardcoded.
Can you guide me on what should I do to give a dynamic Excel input and a dynamic column for the count on the Group By transformation that performs the row count?

Thanks in advance.

Oscar