This…
… thus you can’t force plain data into it’s DB.
… is simply false. As is this…
As well as I’ll probably not be able to just do a query on which I do a query of …
The wpdb class, instantiated as the global $wpdb by the Core, can both insert and retrieve data with essentially pure SQL input. $wpdb->query() will run any query you like, while $wpdb->get_results, $wpdb->get_col, $wpdb->get_row, and $wpdb->get_var will retrieve data according to any SQL that you are capable of writing.
The question does arise as to whether you should run pure SQL, and generally when there is a Core function that can handle the case you should use it instead, but that is a bit of an aside.
If you need to access a completely different database you can instantiate a second wpdb object, just pass it the correct connection information.
And, WordPress is a PHP application. PHP works within WordPress just like it does outside of WordPress, that includes all of the database features. If you need to connect to an MSSQL or PostGreSQL database you should still be able to do so just like normal with PHP.
As far as inserting your data, what I would do is:
- Connect to the other database with a
wpdbinstance. - Grab the data
- Look over the ACF code to work out how ACF inserts data
- And figure out which functions/methods are used
- Format the data accordingly
- And let ACF functions/methods insert data
I don’t use ACF but I have browsed its code. It is a complicated animal. That research is well beyond anything I have time for in connection to this Q/A site.