If you wanted, couldn't you transfer the GRAS Harman Target to the B&K - measure a bunch of different units of various headphones that you've equalised exactly to the Harman Target using your GRAS rig, then measure them all on the B&K, then take an average of those plots on your B&K and that would be your B&K Harman Target?
That's exactly what Harman themselves did to "transfer" the Harman target to the new 5128 (see
here).
It's also what they took considerable flak for from the audio science community as this method is quick and dirty, with less than optimal results.
You just can't
really compensate for differences in acoustic impedance, as the response delta between GRAS and the 5128 will be different for each headphone.
Sure you can find a compensation curve that'll
on average get you closer to compliance, but the individual error for each headphone will be quite considerable regardless.
The right thing for Harman to do would've been to remake the target from scratch.
As a result of Harman's shoddy work and their decision to keep their results under NDA, people like Resolve and oratory were encouraged to do better, and are now working on 5128 targets of their own.