I remember we discussed this before. I tried it with no success but only with the default Piper Cub. Maybe someone can confirm that it is actually possible & if so how it's done.
When I tried it with the Jenny and Trimotor they were invisible both in the select aircraft panel and also in FS02 itself. Looks as though models are backwards compatible (ie 02 planes can go in 04) but not forwards (04 planes can't go in 02)