As per my understanding you want to select only those values in listbox which is specified in ur excel file. It's not straight forward you need to do some work around which is as under:-
You first load you excel file in using mapping load i.e.
'Excel' AS Group
Later in your actual data for the same field you create a group based on above reloaded mapping table i.c.
APPLYMAP('MapExcel', ListBoxField, 'Missing') AS Group
So in this way u will get a extra selection field called group which will have to values "Excel" for data matching with Excel file and "Missing" for the data not specified in excel file. Then simply you can select the Excel group to select values specified in excel.