Is it possible to create a waypoint mission using a desktop PC then transferring the settings to the mavic??

I’ll vote for that. You should be able to safe tasks on the bird.
I asked a similar question here

Hi there, thank you for reaching out and we really admire you being interested with DJI Product. With regards to that Unfortunately we don't any option yet just as you've mentioned. If ever there is this will be posted on our website especially on the release note of the product. I hope you understand. Thanks!

Is there a reliable 3rd party product that I can use in the interim?

Peter_T Posted at 2-4 15:40
Is there a reliable 3rd party product that I can use in the interim?

Yes litchi Hub

Peter_T Posted at 2-4 15:40
Is there a reliable 3rd party product that I can use in the interim?

Hi, thanks for the prompt reply. Sorry that we couldn't recommend any third-party applications as we cannot guarantee the outcome. No worries, we will consider your query as a suggestion and forward it to our designated department for consideration. Thank you for understanding.

DJI Tony Posted at 2019-2-5 14:34
Hi, thanks for the prompt reply. Sorry that we couldn't recommend any third-party applications as we cannot guarantee the outcome. No worries, we will consider your query as a suggestion and forward it to our designated department for consideration. Thank you for understanding.

Any news on that feature?

Yes, but you must use Litchi.  It's an excellent piece of software and makes building and testing waypoint missions very easy.  You can do it anywhere you have access to a browser and it's based on Google Earth.