hi can you tell me is it possible to graft a smaller dicksonia antarctica on to the dead stump of a larger example ?

No - if the stump you want to graft onto is dead, then there's no way for the graft on top to get nutrients, sitting on something that's dead, it'd be comparable to sitting it on a rock or paving slab and expecting it to grow.

ive seen pics of dead TF trunks with ferns in the top, the trunk was drilled out to make a gap big enough to place a pot with a fern planted in it. looked brill..

Ah, well, if its growing out of a pot, then its independent, the dead trunk's just acting as a pedestal. But do you mean an ordinary fern in a pot rather than a tree fern, YDD?
-19 would mean death, wrapped or not, unfortunately, Chrisl... That particular winter saw off a lot of mature tree ferns and rather more Cordylines.