I would like to add my vote to the ability to manually add flight plan addressees.
I fly from a farm strip and regularly copy my flight plan to a nearby controlled airport (even though it's not on my route) so that I can call them immediately after take-off and ask for my flight plan to be activated - it helps them a lot if they have already received a copy before I call them.
I cannot use SkyDemon flight plan submission in this scenario.