No, they cannot be bred because all rescued bears are automatically property of the U.S. Fish and Wildlife Service, and subject to the Marine Mammal Protection Act. Basically, it was a law implemented decades ago to protect marine mammals from being taken in large numbers for the purpose of zoo exhibits; not being allowed to breed them would serve as a disincentive to take them in the first place. But in practice, it just hinders conservation efforts, of which zoos are a big part. Those rescued individuals -- who are forced to go to zoos because they often cannot survive on their own in the wild -- can contribute valuable genetic diversity to SSPs, thereby sustaining a viable insurance population in captivity. Moreover, most zoos don't even have polar bears anymore because there just aren't any left; contraceptives given to females in the 90s/early 2000s permanently sterilized most of them, and thus cubs just aren't being born anymore in most facilities.
My zoo (STL) built a new exhibit (Polar Bear Point) specifically for polar bears in 2015, but weren't even sure they could get any bears to display by the time it was finished. They built it to be able to hold a male, female, and up to three cubs, and intended to procure a young breeding pair for the exhibit opening. But none were available from any other AZA facilities, so they were very lucky to land Kali, an orphaned bear whose mother was shot by Alaskan natives in 2013. They knew that by taking Kali, they wouldn't be able to get a female to breed with him, but it was between no bears at all, or a lone male who would essentially always be all alone.
