Yes, you can easily do that. (Link on how to import)
Just choose "Add new records" and it won't overwrite anything.
Feel free to create a new database to test it out.
If you want the excel sheet imported as a new table instead of adding to an existing table that helps, as well. The advantage to that being that once it is in FileMaker you can script it to import only certain fields from the imported excel table to another table (and do formatting along with it).
I'm having a similar problem with FM Pro Adv v13 importing Excel data into an empty table. When I select the file to import, the target table does not appear as a selectable item in the list (all file names are greyed out except for the option to create a new table. I'm trying to import into the MacrTable shown in the list of tables.
I have not had this problem in the past with FM Pro Adv v12.