Follow these steps to install a grid file so that FME will recognize the file:
Copy the new file to FME's Reproject\GridData subdirectory.
Select Tools > Options and click the Coordinate Systems icon.
Select the applicable datum shift and click the Edit button.
A dialog displays the files already recognized by FME for the applicable datum shift. To include new files, click the Add button, browse to the applicable directory and find the file in Reproject\GridData subdirectory.
Select the file and click OK.