If you use a biodegradable litter then its perfectly fine to use the toilet waste and used litter from your rabbit as a composting material. Unlike cat and dogs rabbits diets are mostly fibre and vegetables so their poops are perfectly fine for composting.
Even if it has some pee mixed in with it this will be fine. You can check if its biodegradable by asking at your pet store or check the packaging. We recommend the paper based litter rather then a wood based litter.