Hello Garion,
A selective migration is possible within pgAdmin assuming that both your new and old databases are imported on the same instance of pgAdmin. However you wouldn't be able to run this on the cloud and I would advise against it if you're not familiar with pgAdmin queries.
You can work around it through the db replication module and additional MFs to sync your desired data.
Here's an example of a workaround:
-You could add a new boolean for your migration named "IsMigrated" and set it to default to true, where you would define it to be false in all your creation microflows and current data. This way when you migrate data via the DB migration you will have all new records with a IsMigrated tag against them on which you can filter and process them following the migration.
Hope this helps.