There is no documentation on manually just extracting tasks on this free community forum. You carn’t normally just copy parts of the backup.zip hidden database directly into another database as you could risk possibly corrupting the entire database doing that.
You can however do a partial restore from a backup.zip or hidden database folder “to then manually extract / copy your tasks out of that” to then incorporate into another database as in my partial restore mail example post. Or see this offical eM Client blog where part way down it also explains how to do Partial restores.
Apart from that if there is a way to just manually copy the tasks part of the database directly to another database without doing a partial restore, then you would need to contact Offical support.
So if you have a current active paid Pro, Personal or Business version, go to the VIP support page and login and lodge a support ticket for assistance.