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.
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.
There is out of the box support for opening a dialog from custom action. Add following attributes to the CustomAction XML node --
For more information go to my blog here. Hope this would help you.