As far as I've looked, there's no way to do that. The only way to hack the DRM back in is by using XperiFIX, but I'm not sure about that
I have tried that by downgrading to 50.1.A.10.40 and used xperifix 3.1 but I cannot confirm any changes.
DRM Info still shows DRM L3 unprotected instead of L1 like before unlocking the bootloader and the camera works fine even without DRM keys, so no changes here either.
Well I got gifted a XA2 by a friend which I am quite happy about it as was curious if DRM keys could be backed up as they could be with the X. Ohh Xperifix is a paid software :S
Backing up the keys is most likely a lost cause unless you are planning to sell your XA2 within a year.
There are more reasons why we want to back up DRM keys, the device guarantee is voided, plus the fact that sony devices with SFOS seem to have a yearly cycle?