The most important thing is probably not whether or not you are qualified in the field, but how you encoded the data.
Certainly in a scientific report using routine data like this I would expect to see a section on the encoding, which would include details of who did the encoding (how many people, what their broad academic background is) and then more importantly exactly how was it done: did you generate lists of codes corresponding to certain groups, did you download them from elsewhere, how were difficult or edge cases resolved etc. That way the process is honestly and reproducibly reported for people who might use your model or your product.
For what itsit's worth, in my field (epidemiology research) we usually have non-clinical people derive lists for our clinical coding, but they do have support from clinicians who can advise on difficult cases and review the lists when done.