It should indeed be possible to use the output of a Microbit as an input on the Crumble. I believe that the voltage of the Microbit is around 3v, and this should be enough to trigger the ‘HI input’. However, if this doesn’t work, you may need to take the analogue() of your chosen input pads, and then set up a conditional statement e.g. ‘If analogue(A) > x’.
I hope this answers your question, and if you have any others, feel free to ask.
p.s. It would be great to see your final project!