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Last tested: April 2021
No. Even PDT Override doesn't allow for this.
The challenge here is that Looker would have to issue a multi part query: first to the database with the PDTs, and then to the database with the actual tables. Writing a coherent query with joins and such that involves two different databases is outside the scope of our product at this time. The closest we can get is to "join" two different queries on the frontend using Merged Results.