I want to create a site action menu item Custom Action which opens a Sharepoint 15 app (default.aspx) as modal dialog. is it possible? thanks.
Is this what you're looking for? http://blogs.msdn.com/b/richard_dizeregas_blog/archive/2013/03/10/leveraging-sharepoint-dialogs-in-apps-for-sharepoint.aspx
You should be able to do it through the following way:
1) Create a SharePoint page: placeHolder.aspx
2) Create an AppPart/Client WebPart for your App and place it on placeHolder.aspx
3) Open placeHolder.aspx inside the Modal Dialog
4) Open the Modal Dialog from a custom action.