Yes, you can use the Flat File templates for this purpose. Are you planning on doing this as a true 1-off import, or do you plan on having additional imports of your iMAC data in the future, as inventory for new iMACs is captured? If you are going to have future imports, then the best way to perform this is to use the Flat File Import connection on a Beacon. Just put your Excel templates in a folder and create the Excel Flat File Connection on the beacon and point it to the folder where your Flat Files exist. Then, just place updated Excel template files in that folder and the updates will automatically be picked up through the beacon.
NOTE: By creating Inventory Devices for your iMACs, they will consume against your FNMS/ITAM License Count.
