Hello Heidi. Thanks for reaching out with this excellent question! Unfortunately, I don't believe the situation you have described would be deemed suitable under the scope of any legislation or guidance for fire doors or fire safety in blocks of flats.
If the door was non-latching and you were simply fitting handles to the surface then you may have been fine, depending on the door's certificate and the installation method, but even that isn't guaranteed. However, the mechanism for a latch is fitted inside the door leaf and the associated handles attach to it through the leaf, which thoroughly compromises its integrity.
This is also unlikely to be accepted. Modification to a fire door leaf / set must be done by a competent person (i.e. knowledgeable, skilled, and / or experienced with working on certified fire doors) and only in accordance with the manufacturer's instructions and the certificate for that particular door leaf. Fitting a fire rated latch and handles in accordance with such is fine, but fitting a non-rated mechanism would not be in line with the certificate or instructions, and replacing such a thing with a fire rated mechanism could require removal of more material and back-filling previously removed material, unless the mechanisms are identical in size and shape, which would further deviate from the original certificate and further compromise the door's integrity.