How could it create records in the related tables? How would FileMaker know to create 1, 2, 3 or x related records for each record imported into the main table? What data, other than the foreign key could it import into the related tables?
In FileMaker, you import data one table at a time and you can do this by hand or through a script. To do what I am understanding you to want, you import the records from the main table of the source file into the main table of the target file and then from a related table in the source file to the corresponding related table in the target file. You then continue this process until you have imported all the data you want to import.
I'd need to know more about how the data is set up in the spreadsheet to answer more specifically to what you are trying to do.
In general, you can import from a spreadsheet into Table A, selecting the fields that belong in the parent record. You can then perform a second import of the same spreadsheet into Table B, selecting the fields that should go into Table B as long as you also are able to import the column that has a value that can be used as a primary key to link the records in Table A to related records in Table B.
Example, you have this spreadsheet:
PrimaryKey Name Address
1 John 555 somewhere drive
2 Jane 333 anywhere street
You want to import the data in to a "Names" table and also an "addresses" table.
Go to the layout for the "Name" table and select Import Records to import records from the spreadsheet. Select the first and second columns to import so that you import the primary key and the name. Now switch to the layout for Addresses and import again, this time importing from columns 1 and 3. If you have defined a relationship in the database to link the primary key field of the Names table to foreign key field of the addressses table (where you imported the number from column 1), you now have your data imported and linked.