I don't think that is possible at the moment.
As far as I understand you need to create the app already as an Atlas UI app. (that may change in future versions)
You might have a chance to create a new AtlasUI app and merge your existing modules in there, but I would really be carefull with such an approach.
Currently the Web modeler is only enabled for newly created Apps based on Mendix Atlas.
Thanks! I created a new app with Mendix Atlas and added my modules. Seems to work fine.