I do not own a DJI Fly drone with waypoints so keep that in mind when reading my answer...
From what others have written, I have concluded that you cannot pre-program a DJI Fly waypoint mission with negative height values using DJI Fly. The...
You do not have to fly to create a mission. You can create a mission by placing waypoints and POIs on a map, then save the mission so that it can be flown later.
Yes, if you are not in the USA, your missions will sync with the "cloud" so that...
If you are asking if a Litchi waypoint mission can be converted and loaded into DJI Fly, the answer is "yes". The conversion software and instructions are available on this site:
https://www.litchiutilities.com/