Halna outliner is my primary Android app, I have found no better way for organizing my lists and notes. I would love to have Libre Office offer similar funcionality.
PS I know that outline like documents can be created in LibreOffice using heading styles, but this is a cludge, compared to the ease of editing with Halna Outliner.
I never use ‘Outlook’, but…
As far as I can see .hol files are simple text files in the manner of csv with three columns separated by commas. (In addition the data rows may be divided in sections in the same way as old .ini files are.)
You may import such a file with ‘Calc’ and edit the content in whatever way you want. However, neither a text processing program nor a spreadsheet program will speciffically support the handling of calendar data. ‘Word’ ™ and 'Excel’™ surely also don’t.