It is not easy/straightforward, and there are limitations.
For ARM devices there is a “better” solution through a container:
http://forum.mikrotik.com/t/serial2http-container-to-bridge-serial-to-the-routeros-cli/164589/1
the same thread contains some hints towards the possibility of using /interface/ppp-client/at-chat:
http://forum.mikrotik.com/t/serial2http-container-to-bridge-serial-to-the-routeros-cli/164589/1
But it has to be seen how it works in practice on the Knot.
Check also:
http://forum.mikrotik.com/t/how-script-sending-an-at-command-to-a-gsm-modem/175576/1
https://wiki.mikrotik.com/Sending_text_out_over_a_serial_port
http://forum.mikrotik.com/t/lte-at-chat-command-formatting/144459/1