The easiest way would be to create a blank app and then add a startupFcn.
In the startupFcn you can pass in the app.UIFigure to your exisiting code.
You may need to slightly modify your exisiting code.
% Code that executes after component creation
function startupFcn(app, varargin)
yourexistingfunction(app.UIFigure);
end
You can add the startupFcn by clicking the App Input Arguments. You can use varargin as you cant leave it blank.
