At the moment it is not really possible to globally predefine Mini-Factsheets.
There is a default xml mini factsheet and there are several specific mini factsheets for some item types.
The default xml min fact sheet is the basis for the definition of the mini-facthseet in Components in SSC. So it would be possible to add properties either in xml or in Components in SSC.
When editing the mini factsheet for a specific item type the predefined properties "disappear" and need to be defined newly.
And the xml mini factshees would needed to be edited, too.
So there is basically no way to predefine properties for the mini factsheets for all item types. It would be good to have a better support here as it is lots of effort to change every single mini factsheet.