Since you know what the name will be every time:
If you are pulling the data from the AS/400 in the CL you can pull the data with the FTP script and then rename the file after exiting FTP.
If you’re pushing the data from the client, in the FTP script try using SYScmd to call a program on the AS/400 that will rename the file.
If you’re executing FTP interactively, you can type any name you want obviously. If it’s a batch process, then your FTP commands are being read from a script/file. And as with every file, you can update, add or delete any record in it by any file maintenance procedure you want to use.
Where’s the problem? Usually a simple SQL UPDATE statement with a WHERE clause to recognize the MGET record is all that’s needed. Or generate a custom FTP script from template sections by using a Qshell script. Or simply write the FTP subcommands in a basic RPG or COBOL program. Or even a REXX procedure.