The file including the user dicts is there:
<CONFIG DIR>/calibre/dictionaries/prefs.json
You may copy it from a PC to another.
You can include a list of words with their language directly in this file (calibre must be closed).
You can also use a basic python script to include the content of a file (word + lang) into this file, you'll need to use dump() from the lib "json"
Or, if you use the python prompt opened by calibre-debug, you can use the utility from calibre :
from calibre.utils.config import JSONConfig
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