Can I copy my post from steemit as an answer to my own question for musing.io?

I would say it's better to re-word it and link to the original because:

(a) Musing is typically for shorter and more precise answers to questions allowing users to scan through and find the best answer to the question. Steemit blog posts would typically be too long for the purposes of the musing audience.

(b) You won't get problems with cheetah if you re-word.

You can then link to the original post to give people more information if they want it.