@okkebal said:

There are two things missing from the Specials: "The Reunion - Lunch with Joss, Nathan, Alan and Ron" from the Blue Ray and "The River Tam sessions" where Joss (as an Alliance Scientist) interviews River.

I cannot add those to the specials episodes.

If they only appear on the DVD/BluRay I don't think they really qualify as a separate 'entity'? I could be wrong though? The weird rules here are baffling at times.

All the stuff under "Special" is from the extras of the DVD/Blueray. There are also some "deleted scenes" that are missing here.

I'm sure there are several versions of the blurays, all having different extras or special features. But in my case, the River Tam Sessions are on the Serenity bluray extras, not the Firefly disks. And as far as I know there's no way to add extras to a movie here.