Hi Sugarplumsss,
I would record your song first in MIDI on the keyboard, and then upload it to a DAW, where the MIDI channels can be separated out, edited, and then converted to audio. I'm not sure if Audacity has those capabilities. There are entry level DAWS that don't cost a fortune, that can handle the task.
The reason I suggest MIDI first is because of the editing capabilities. You can't come that close to fine editing when all you have is audio tracks.