This is already possible, you can activate 5.x with a newly purchased key.If you can't develop a version of Radioboss for Linux, I recommend selling licenses for 5.X versions (and allowing licenses for 6.X versions to be used in 5.9 too), to avoid the only alternative to using Radioboss on Linux, either using the hacked program or illegally.
Latest 5.x can be downloaded here: https://djsoftnet.akamaized.net/radioboss_setup_5.9.4.0.exe