If you can establish an ODBC connection to your excel file, that may be the simplest way to go. I haven't tried an ODBC link to Excel, however.
Another option is to import your data into a new, temporary table and define a relationship from it to your Customers table that matches by customer name. You can then import the data from Excel to the temp table, perform a find to find only those records that match to a customers record and then import that found set into the final table.
Sending data back to an existing Excel file, unless the right type of ODBC link can be set up, would be difficult to do. It might be possible to output the data to a text or excel file that can then be imported into the Excel file via a Macro in the Excel file.