Quick tutorial to get around the annoying issue where Adobe Lightroom CC doesn't let you to export to anything other than PS.
1) Open Automator and create a new folder action
2) Create or choose a folder (mine is called Edit in Pixelmator)
3) Choose or search 'Launch Application' from the list of choices and choose Pixelmator Pro
4) Choose or search 'Open Finder Items' and choose Pixelmator Pro as the destination
And Voila! Auto-open in PP. You can't yet write back, nor can you get presets to transfer over with the DNG file - @Pixelmator - that would be great addition.