To further expound, it depends on what "kind" of infertility. If the dams can produce fertile eggs but the embryos can't implant or there's a placental defect, you can harvest the eggs, do IVF, and implant in another dam. If the males make sperm but have vascular issues that make them impotent, you can treat with drugs (like Viagra) or harvest sperm for insemination/IVF. If the mice can't produce gametes, you can use "conditional knockout" like the Cre/LoxP system so the gene is present and working until you're ready to make the test animals, and then either knockout the gene in the whole animal or in the specific tissue you want. I've also seen temporary "rescue" of a knocked out gene with injecting mRNA at the time of fertility defect to overcome the issue long enough to get viable pups.