For a batch operation such as this, I think your only option is to do it outside of RM using SQLite directly on your database. The easiest solution is to append or prepend the definition sentence or paragraph for the Occupation value found in the Event Description field to the Event Note field. Both fields are in the same table which keeps it simple. The Description field may get truncated at 100 characters in data transfer to other systems so the Note field is preferable.
However, there is no primary ‘definition’ whose changes would ripple through to all instances of that Occupation. It might be preferable to use a Citation instead, one for each different occupation reused for every instance of a given occupation. That’s a bit more complicated as it involves more tables and steps but is do-able and the result might be cleaner with the definition appearing only once in the Endnotes of a report.
Edit: My memory recollected a discussion about occupation definitions was held some time ago. A search turned up my post making the same suggestion about reusable Citations last year and that you were the starter of the topic! See
https://community.rootsmagic.com/t/shared-notes-are-these-possible/10980/12?u=tomh