If you would like to create a Data Transformation grid based on an Excel file that you have already created to use as an Organisation Transformation grid, you will first need to open the Manage Organisation Transformations section of Importacular.
Next, you will need to proceed as though you are creating a new Organisation Transformation grid. You will click on the green circle with the white plus sign/cross and select a Field Type/Code table as necessary (see this knowledgebase on adding an Organisation Transformation grid).
Once you reach the screen where you would build the transformation grid, you will then Import your .xml file. Use the "Excel Import" option (circle in red below) to import your Excel file.
From there, you can follow the instructions in this knowledgebase article for importing Data Transformations from Excel.