Fortunately I had not too much favoUrites to import... For anyone who'd be in the same situation, here's what I had to do to be able to import previously exported favourites in geojson format (Here wego to name it).
After "sharing" the favorites from the GPS app in geojson format, open the file in your preferred text editor and add in each "properties" section of each line: "Name":"the favorite name".
Then convert the geojson file to the MapFactor xml file using this online converter.
You should then be able to download an xml file you'll rename to favourites.xml and move to the location you chose in the advanced/export section of your MapFactor settings.
Last thing to do is now to "restore" (you'll have to do a backup before) the newly created favourites.xml file.
Should be OK. This has worked fine for me (apart I lost the groups, but it's acceptable for me).