I don't think that's currently an option--though I totally agree that it would be a nice new feature.
Which version of FileMaker are you using?
If you are using FileMaker 12, there's a way to do this. Use Insert File to insert a copy of the file into a container field with the "store a reference" option. In that script step, you'll find the options you want to specify the excel file type. Once the script inserts the file into the container field (I use a global container field for this.), you can extract the file path to your inserted file into a variable.
Your Import Records step can then be set to use this variable as the file reference if you enter two file references: Make the first line the name of the file and the second line of text (use the add file button) needs to be a reference to a copy of the excel file that is valid at the time you set up this script step. (While setting up the script step, FileMaker will use the second reference while you map columns to fields. When the script is performed, it will use the reference in the variable so long as it is a valid reference.)