To create an APPEND query, you need to open the table that contains the data you want to append from, select the fields you want appended, set your criteria and point to the table you want to append to.
With that said, "50 tables in a flat file" does not sound like you are using MS Access.
Do you want to import these tables into Access and append them into one table? Where is the "another table in particular file"? Same "flat file" or in an Access db?
Under normal circumstances, you would NOT store SUMMED data... you let your query or report do the summing for you.
There are many ways to accomplish your task, but one would need additional information (outlined above) in order to provide the best or quickest solution for you.