Yes, this is possible. Create the combobox using the Access combobox wizard, selecting the "find record" option. Once it has written the code for you it will be necessary to modify it to work with your subform. Basically, you'll need to redirect all of the "pointers" to the subform. To do this modify it to look something like the following:
Forms!MainForm!SubForm!...
As for finding the customer while typing, you need to insure the rowsource property of the combobox contains the field with the customer name but is bound to the field with the customer's primary key. Hide the PK field by setting it's length to '0'. Set the AutoExpand property to Yes.