the script will work as expected. But you do not need the keyword "add" in the way you (mean to) use it. An "add" in front of a load-statement takes affect only(!) when you start a "Partial Reload" from Mainmenu/File/Partial Reload or with <ctrl> <shift> R. In this case two things will happen: a) the existing tables won't be truncated and new records will be added where "add" is specified. In your case using the script a second time with Partitial Load only the data from "my_db_table_in_ms_access_Northwindmdb" and "[C:\draft.xls]" will be appended (and therefore being twice in your table).
This means that QV takes care of sourcefield names (case sensitive!) and inserts and appends the records of your three sources into one table because all the fieldnames are the same. See Help for more information about the (automatically) concatenate load. This is a powerful but sometimes suprising feature.