The only way to make FTP work out another port is if the FTP server you are connecting to (assuming you're at the client's side) 'listens' on a different port. If you're at the server side, then it's up to the FTP server application if it allows you to configure the FTP service to 'listen' on a different port other than 21.
I am talking about filezilla. According to what you say I think that the solution is to make ftp 'listen' to the desired port via ftp service (IIS manager) of the O/S, and then simply declare it on the ftp application (filezilla). Have I understood well?
Johny, you are correct. Simply configure the IIS FTP service to 'listen' on a different port and then make the necessary port adjustments on your FTP client so that it connects to the FTP server at the same port the server is 'listening' on.