Is it possible to create app for musing.io site?
Musing itself is an app built on top of Steem. Musing currently doesn't have any api and I don't believe Musing has any intention of providing one in the future. So, no it is not possible to create apps for musing website.
In a few years, if Musing becomes a popular website, it might consider providing an api to make bots and other apps that interact with website. Until then, you can't build apps for Musing.
To the question in your title, my Magic 8-Ball says:
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