Making a "plugin" or "hotspot" that can be dragged into place is fairly trivial and can be done with the stock krpano. However this happens just in the memory of the user; if the user reloads the scene, the added plugin/hotspot won't be there anymore, and other users on other machines will not see it either.
If you want the plugin/hotspot to become a permanent part of the krpano scene, you have to somehow alter the xml on the server, or put the information into a database and get the data from the database back into krpano. This is a lot less straightforward, and highly depends on exactly what you want to achieve.