Is it possible to connect ownCloud to ubuntu one over the "External storage support" app in ownCloud. It works pretty well with dropbox and Google Drive.
No. Unfortunately it is not. As you probably have seen, Ubuntu One is not listed in supported protocols in the external-storage dropdown menu in your Owncloud environment.
In fact, the protocol that is used for U1 is not a standard protocol (but Google Drive and Dropbox are not standard either). It is developed by Canonical. It is called ubuntuone-storageprotocol. API's to interface with Ubuntu One are however available so there is nothing stopping the owncloud devs to start supporting this. It just hasn't happened (yet).