Modifying contents of file using TCL in a Upoc thread

I need to open and modify the contents of a file as part of a upoc thread using advanced scheduling.  I specifically need to place a FHS segment at the begining of the file and an FTS segment at the end of the file with multiple messages.  I can successfully open the file with the “open $listedFile w+” command but cannot seem to find how to insert these segments and close the file for further processing with desired modifications.