Your intention is very noble, but it seems to be unneseccary.

According to this study [Outcome of FeLV-infected cats in a large pet adoption program][1] 

> Rates of feline infection with feline leukemia (FeLV) in the U.S. are estimated at around 3 percent, including approximately 60,000 shelter cats. Such cats are frequently euthanized following a single positive test for the virus even if they show no signs of illness.

If scientists were actively searching for cat swith a resistency to FeLV, they would probably be able to find enough of them in shelters.


  [1]: https://sheltermedicine.vetmed.ufl.edu/2020/01/01/study-felv-adoption/