Is it possible to add a new UmbracoApiController in the backoffice tool? We currently have to open project in Visual Studio and add a new UmbracoApiController and then compile the project and replace all files in the host platform.

Hi there! As far as I can tell, this is not possible. The C# code for the controller needs to be pre-compiled and cannot be added while the website is running. If you want to create and maintain an API, you'll have to do that through code and deploy new changes.