Would it be possible to add rtl_433 to the dietpi-software list please?
It is currently available from the testing Raspbian repository.
If not, could there be an inbuilt way to add a repository such as testing (or any other) repository without impacting the normal repository lists and priorities?
echo 'deb http://raspbian.raspberrypi.org/raspbian/ bullseye main' > /etc/apt/sources.list.d/bullseye.list
echo 'Package: *
Pin: release a=bullseye
Pin-Priority: 100' > /etc/apt/preferences.d/bullseye
apt update
apt install <package_name>
But take care about additional dependencies pulled from Bullseye repo. If this is too much, it can damage your system by producing incompatibilities with other pacakges from Buster repo. Pin priority 100 means that packages are always installed from stable repo, if available, and only pulled from Bullseye, if not available in stable repo. apt upgrade will upgrade these packages automatically as well.
But generally, I would recommend to compile this manually instead of mixing two repos. The resulting software/binary will be most current version and compatible with default Buster library versions etc, hence avoids conflicts with any other package.