The FVector2D constructor that accepts a FVector is very handy, cheers!
As of UE4.27.2 there’s no constructor for FVector that will take an FVector2D, so you have to break it up into floats:
FVector2D Vec2D(1.0f, 2.0f);
FVector Vec3D(Vec2D.X, Vec2D.Y, 3.0f);