Questions regarding serial passage of virus through animals arising from the paper The proximal origin of SARS-CoV-2
No. Serial passage through animals attenuate the virulence of a virus In It's ORIGINAL Host. it can cause and will cause an increased virulence in the animals that were used in serial passage, and, if the animals have similar receptors as humans, increase the virulence in humans as well. like the two experiments that passed bird flu across ferrets until they become virulent and airborne across them.
https://www.cidrap.umn.edu/news-perspective/2014/04/experiments-render-h7n1-virus-more-contagious-ferrets
https://www.scientificamerican.com/article/pandemic-bird-flu-studies-public/
https://science.sciencemag.org/content/336/6088/1534.full
https://jvi.asm.org/content/88/12/6623