Tinderbox imports OPML files dropped onto Tinderbox views, and also reads OPML outlines.
If an OPML file has extended outline attributes, and if the extensions do not conflict with Tinderbox attributes, then Tinderbox will make a user attribute for each of the extended attributes. The default 'text' attribute is mapped to $Name [sic]. The non-standard '_note' attribute popularised by OmniOutliner is, if found, mapped to $Text with ' ' being parsed as paragraph breaks.
The user attributes will initially be defined as Displayed Attributes as well, allowing for easier inspection of the extended data.
OPML support includes Projekt extensions.
The non-standard but generally used OPML '_status' attribute data is mapped to $Checked which of course can be displayed in outline view as a tick box (good for 'to do' lists).
OmniOutliner exported OPML: see further detail.
Note that the OPML <expansionState> element is not supported. By default, any new container notes created from the imported OPML data are all collapsed in Outline view.
Tinderbox will expect dates to be in RFC 822 form in accordance with the OPML spec: see more.