According to this thread, the answer is that the DJI FPV system doesn’t natively support BuddyBox.
However, they do mention that you may be able to use an R-XSR, and feed the SBUS output of the Air Unit into the SBUS in pad to use it as a backup receiver.
The difference is due to regulation - in the EU, the permitted radio power output is lower than that allowed under the FCC regulations, which results in reduced range.
Without a licence from your countries telecoms regulator, it would be illegal to operate the FCC equipment in Europe.
It is likely to be a software setting, but there might be hardware ...