I want to enable SSH service on BusyBox server (currently I can connect only via Telnet), so I think , that I need to compile SSH server , and send it to BusyBox router. So my question is - how to create SSH server binary to send it to BusyBox via FTP?
You're taking a wrong direction.
1/ "How to enable SSH on BusyBox."
You can't.
2/ And "without recompiling BusyBox?"
You won't, because #1.
Because busybox in itself does not provide an SSH server nor client.
Now that your question is answered:
The usual way to add an SSH server to a busybox based system is to add dropbear, first on the list of "tiny utilities whose functionality is not provided by busybox":
https://busybox.net/tinyutils.html
https://matt.ucc.asn.au/dropbear/dropbear.html
How to compile dropbear (because you will have to), then install and run on your system is now another topic, on which no one can help you without further details on your target system or your cross toolchain.