Considering that both .fbr and .fbz files come from BB FlashBack software, if any kind of fbr to fbz conversion is possible it would be done with the software.
Unfortunately, the modern version of BB FlashBack no longer uses .fbz files for any kind of content, so this is either a typo, mistake or simply not possible anymore.
Blueberry software offers older versions of BB FlashBack tools for download on their website, so perhaps this is the only way how this can be done.
Updated: October 26, 2023