If you can set up a named range in the excel spreadsheet, then you might be able to import just the records in the named range. I doubt that will work for you here though.
It is possible, I have read, to use ODBC to link FileMaker to an Excel File as a data source for your database. I'd check into that option as you can then use one of two methods for importing data from the ODBC data source into a table in FileMaker. I seem to recall that at least one method uses a SQL query so you should be able to limit which rows of data from the excel file get imported.
Thank you for your answer, I'll see if I can find out more about that. I'll post back here if I do!