OK, here's one way.
1) Create an integer variable in your SSIS package - say MaxId.
2) An an ExecuteSQL task which connects to your target db (the one you are importing to) and does something like this:
select MaxId = max(Id) from dbo.Customers
3) Configure the ExecuteSQL task to return the result into the MaxId variable.
4) Create a data flow task and add an OLE DB source. Define the connection as your source db.
5) Set the data access mode to SQL Command.
6) Write the SQL command text, using a parameterised query to select [column list] where Id > MaxId.
7) Connect the OLEDB source to a suitable configured OLEDB destination.
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