Each user can only like each individual post once. So if you wanted the @system automated forum maintenance user to give @johnwasser 2000 hears, then you would need to created 2000 dummy posts under @johnwasser's account, each of which the @system user would give the allowed one heart. Repeat that for tens of thousands of users who had karma on the old forum. So the number of dummy posts required would equal the total of all karma counts of all forum users.
Alternatively, you could create one dummy post for each user to contain all the karma migration hearts, and then create thousands of dummy user accounts to each like that single post in each user account up until it reaches the number of hearts to match the karma count. So the number of dummy users would equal the maximum karma count.
Or perhaps there isn't a hard requirement that each heart be associated with a post or that each user can only heart a given post once. I'm only speculating that the post:heart:hearter association is built in to the structure of the Discourse database and/or code.