No, the ADS-B barometric is always referenced to 1013mb regardless of height. So you would need to apply QNH for all ADS-B reports.
This is always going to be approximate. Even assuming you have access to all the QNH data it is not super accurate - it may be a worst case period forecast for a region. Then the deviation of the real atmospheric conditions from the Standard Atmosphere will also add error.
Where it is available use "difference from barometric" in the ADS-B velocity reports as this will give you a true measure of GPS geometric height to the best accuracy the aircraft can provide - which again is not super accurate but perhaps better than the multiple approximations that the barometric conversion will give you.
The barometric "foot" is a somewhat flexible animal. It is useful for safe separation because of its universal applicability - and to do that it only needs to work as a relative measurement over quite short distances. It is not great as a surveying tool though.