We are planning to develop webApps using ArcGIS Online/Portal and publishing in different browsers/mobile devices. In this regard, We segregate as two modules.
Module No: 1
Use the Templates which is available in ArcGIS Online and publish the apps.
Module No 2:
The newly created apps can be uploaded to ArcGIS Online/Portal accounts. As well as We can download the existing templates from the ArcGIS Online account, modified according to needs and reupload to ArcGIS online account.
We want to know Module No 2 approach is possible or not?
As Domenico noted in the post above, you cannot upload custom apps back into ArcGIS Online or Portal for ArcGIS. What you would do is host your custom web apps in your own web server, then reference these custom apps by adding them to ArcGIS Online or Portal for ArcGIS, by registering them as a new item.
What this means is that your ArcGIS Online or Portal for ArcGIS members will be able to find and discover your web apps, but when they click the item, behind the scenes they will be redirected to the location where you are hosting the web apps.
Hope this helps,