Comentário oficial
As technology has improved, we've been able to provide support for more formats, including lossless WAV and AIFF, and on the CDJ-2000NXS2, we added support for FLAC and ALAC.
There is no transcoding export engine in rekordbox, such a feature would only lengthen the export process, and while I understand that you'd love to simply click-and-export to a universal format, unfortunately there are also additional technical hurdles which may transform your nicely aligned cue points and grids for your FLAC file to slightly different places on an MP3. Personally, I'd rather Pioneer DJ not include a feature that made things worse by trying to make them better. Yes, it's more time consuming for you to create a second version of that file, then check the beat grid, create your hot cues, memory points etc. again, but if it means they'll be accurate, it would be worth it.