I don’t know if extinct viruses are used. But viruses can act as a vector to modify the DNA in a cell I am sure dead viruses are used to avoid an infection.
Copied from www.nytimes.com Oct 2017
In July, scientists reported that a strange protein courses through the veins of pregnant women. No one is sure what it’s there for.
What makes this protein, called Hemo, so unusual is that it’s not made by the mother. Instead, it is made in her fetus and in the placenta, by a gene that originally came from a virus that infected our mammalian ancestors more than 100 million years ago.
Who knows how these genes can be modified…