How to turn an existing application into a shopify app?

[sam1905]

I have an existing web application with ruby on rails and I think it would be perfect as a shopify app. I did everything (parnter account, development store, etc.) and when I created a test app with shopify app create rails , I was able to add the app to my store. However, how do I do that when the project already exists? When I try to run shopify app serve I get the error message

┏━━ Error ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
┃ ✗ You are not in a Shopify app project
┃ ⭑ Run shopify app create to create your app
┃ 
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

But if I were to run this command, it would create a new project, which I don't want since I already have the project. I only want to turn it into a shopify app project, but I haven't found an answer whether this is possible and if yes, how to do it.

I hope my question is clear.