OK, it is possible, but it's messy. The Pup and Tripe can be added just by editing "Aircraftinfo.rofcampaign" in the input folder.
Plane
{
Id = 102020;
Desc = "Sopwith Pup";
RoFType = "soppup";
Purpose = 1;
Flyable = 1;
Role = "Scout";
}
Plane
{
Id = 102021;
Desc = "Sopwith Triplane";
RoFType = "soptriplane";
Purpose = 1;
Flyable = 1;
Role = "Scout";
}
And by manually updating the Planesowned.config in the user folder.
Problem is that although the Campaign will add them to the missions, they don't show up in the briefings.
Enemy Forces
RNAS 8 flying
1 RFC flying Nieuport 17
RNAS 3 flying
RNAS 1 flying
57 RFC flying Breguet 14
16 RFC flying Breguet 14
12 RFC flying Breguet 14
40 RFC flying Nieuport 17
Enemy balloons are in your area
Notice the blanks, it's the same when you try to report the mission too. Just a space where it should say Sopwith Pup or Sopwith Triplane.
So not really practical, we need Pat to update really.