First of all, koalas are not bears, they are marsupials. The giant panda is a member of the bear family, Ursidae, but it diverged early in its evolution from other bears, and is placed in its own genus, Ailuropoda. There is no possible way these species could breed – you might as well ask if a kangaroo and a horse can breed. For two different species to interbreed, they must be closely related, usually within the same genus, and at the very least within the same family – horses and zebras can crossbreed because they both belong to the genus Equus. Even in these cases, the offspring are almost always sterile. If two species can interbreed and produce fertile young, it shows that they are very closely related. For example, the grey wolf is the direct ancestor of the domestic dog, and the two are still so close genetically that they are able to breed and produce fertile offspring.