I don't know how OLE DB handles DBFs and DBCs or stored procedures. But this is a best guess.
#1. If you have your table connected to a DBC, set the Default value to "", 0, .F. or whatever you want. When you insert a record without the field, it will be defaulted. If you insert a record with a value, it will overwrite the default.
#2. If your table is not connected to a DBC. Connect it and see #1.
As for General fields, maybe you can use a stored procedure with a default xls file.