Autoincrement When Importing

I have a database that a client of mine has started to create. He wants to be able to import data from an excel spreadsheet into a table in Access. I created a macro that works fine but now I am having an issue with importing additional data.

His excel sheets consist of Agency ID numbers, first name, last name, SSN, etc. He also has a column in the table called order number and internal ID. Which I duplicated in the spreadsheet. He wants the internal ID to autonumber. When I first use the macro to import the spreadsheet into Access it numbers the rows accordingly (after I fill the excel sheet using the series option). However, when I try to import an additional spreadsheet (which is a test sheet)I have to fill in the Internal ID number manually and consecutively according to the last record in access.

The sample spreadsheet he sent me has 2079 records which imported fine with the macro and then I made a second spreadsheet with two rows with information in them and if I don't put in the numbers 2080 and 2081 it imports them at the top of the table without any number in the internal ID. I hope this is making sense.

Can anybody help me make it so that it automatically puts a number (consecutively)in the internal ID field when I use the macro to import another spreadsheet.

Thanks in advance

Tracy

P.S. When I go to the order details table in design view it says that field is autonumber. If I do not assign a value to the Internal ID field in the spreadsheet it comes up with an import error saying that the fields had a null value but if I number the internal ID field manually in the spreadsheet it imports just fine. I don't want to manually input numbers into the internal ID field in the spreadsheet, I want access to do that for me but it won't let me.