i’m not really familiar with the bulk import process, but just based on the rules noted on the import page, it doesn’t look like the bulk import itself allows for the simultaneous import of observations and related observation fields.
that said, you could probably load the observations, then go back and batch edit your observations, manually adding the necessary values.
the only way that i think regular folks like us would be able to import both observations and observation fields in an automated way would be to use the API. (how comfortable with coding are you, and which languages are you most comfortable with or would you be willing to learn?)
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someone else might know of a more automated way to do this without coding though – so maybe see if someone else responds. the only other thread i can find where someone has asked a similar question seems to have gone unanswered though: https://forum.inaturalist.org/t/import-via-csv-can-this-include-additional-observation-fields/13013.