Do you know a tool that would exploit all the option of omnioutliner
type of OPML files ?

The characteristics are that it uses more than one attribute for an
"outline" tag : in addition to the 'text' one, there is a '_note' for
... some notes associated to a line of the outline and indeed any
number of others that are translated within omnioutliner into columns.
Other options are the inclusion of links in the notes.

With 'OPMLmanager', I successfully imported such a file, but then lost
the additional attributes (and of course there was not way to edit
those data.

Being able to add that sort of level of complexity would be great !