How do I override the dates with Temporal tables?
The time in a temporal table is "system time" not "business time".  That's the distinction that the feature name "System-Versioned Temporal Table" is getting at.

You inserted the data today, so it's versioned as of today.  You can use the temporal table to track the changes you make going forward, and retain change history, but it doesn't replace a slowly-changing dimension for business analytics.  As you noted, Temporal Tables belong in the "Data Lake" or "Raw" or "Staging" layer.  

If you want to add additional history representing changes to the data before your initial load, you can turn system versioning off temporarily while you load data directly into the history table.