sftp or scp (secure copy) can be added to your AS/400 in a number of ways. You could buy a commercial version, I am sure IBM has one and there are other vendors like Pro:Atria
There are also open source versions available from the gnu project and others. You will have to have a compiler (probably C) and then download the source code from your favorite open source site (try sourceforge.net) and then compile.
If you have any opensource gurus in your organisation I am sure they can do this for you.