Unfortunately it’s not very trivial. At this point we haven’t studied in detail the formats that DayOne accepts, we hope to get to that at some point so we could potentially have a dedicated import/export, but unfortunately it’s not at the top of our priority list to be perfectly honest. You could also ask the guys from Day One, perhaps they have solutions.
Finally, one thing to explore, a quick google revealed that Day One does seem to support a URL scheme:
One solution might be to explore exactly what I also did yesterday for Apple Notes:
See for example this article I also came across:
https://thesweetsetup.com/make-journaling-day-one-even-better-automation/
(latest rumours are that the Shortcuts app might also come to the Mac…)