I don't know about STeem, since I haven't used it for a few years... it probably has some mechanism for remapping keys.
Hatari allows you to define a custom keymap file, which maps SDL keyboard codes (which you can find online, in one of the SDL header files at least) to ST keyboard scancodes.
About the keyboard -> joystick mapping, I'm not sure... if not, you might be able to configure your joystick software to simulate a certain keypress when a particular button is pressed. Maybe you can find a generic joystick macro program if one didn't come with the device.
tá'n poc ar buile!