>>32150766Chuubanite would be one answer, but even then you want it to be something that doesn't cause hereditary gene manipulation, because your DNA would still start to vastly change over just a couple of generations to accommodate the tail and ears, as well as possibly other features, and you are back to square 1.
If we go with it I would suggest that risuner traits are completely artificial and caused by their chuubanite. This would mean a few things, and we can kinda handwave some others.
1. Each risuner would be born a normal human, no tail, no ears, just a homo sapiens.
2. They would gain those traits in the first few years of their life due to exposure to the chuubanite
3. We still retain some genes for a tail, so that would be very much in the scope of what can happen.
4. The problem happens when you consider the next generation. At this point your DNA would already be changed, so you would once again be practically sterile for other species. We can get by that by recording some extra info in your genes that, at the time of birth, store your DNA in a cache, and leave it alone when the chuubanite effects start kicking in. This way you keep the squirrel traits and still can breed with other species normally. This would probably just amount to a small extra sack on your sack that would store that genetic information.
5. If we go with that, the resulting baby from breeding with any other homo species will always be a normal human, and would have to gain the squirrel traits by exposure to your chuubanite.
Honestly this is all very, very fucked and I hate it, but it's a way to achieve both effects that you want.