[Answer]-Is it possible to manually set a django foreignkey ID to an item that doesn't exist yet?

To be able to handle any database that Django supports and not have to deal with peculiarities of the backend, I’d export the old database in the format that Django loaddata can read, and then give this exported file to loaddata. This command has no issue importing the type of structure you are talking about.

Creating the file that loaddata will read could be done by writing your own converter that reads the old database and dumps an appropriate file. However, a way which might be easier would be to create a throwaway Django project with models that have the same structure as the tables in the old database, point the Django project to the old database, and use dumpdata to create the file. If table details between the old database and the new database have changed, you’d still have to modify the file but at least some of the conversion work would have already been done.

A more direct way would be to bypass Django completely to do the import in SQL but turn off foreign key constraints for the time of the import. For MySQL this would be done by setting foreign_key_checks to 0 for the time of the import, and then back to 1 when done. For SQLite this would be done by using PRAGMA foreign_keys = OFF; and then ON when done.

Postgresql does not allow just turning off these constraints but Django creates foreign key constraints as DEFERRABLE INITIALLY DEFERRED, which means that the constraint is not checked until the end of a transaction. So initiating a transaction, importing and then committing should work. If something prevents this, then you have to drop the constraint before importing and add it back afterwards.