I haven't done this, but it should theoretically be possible. I'd start
with a telnet client and see if you can feed it text strings and get
responses just as if you were sitting at a telnet terminal. Once you get
that working, just use a command line ftp client on the telnet server.
Download files to the telnet server, and then to the client from there.
Streaming it in one go might be trickier.

I know this isn't a direct answer to your question, but it might help
you find a better solution to your problem.

It sounds to me like you're trying to transfer files between two
hosts, neither of which is your localhost. It might be worth knowing
that the FTP protocol already supports this kind of transfer, i.e. an
FTP client on one host can be used to transfer files directly between
two other hosts, without the files passing the client host.

All that's required is that the client establish control connections
to each of the remote hosts. It's described in the RFC, in particular
figure 2 and section 5.2.

/gordon