Insert into PosgresTable (f1, f2, f3,) Select f1, f2, f3 from
AccessTable
It seems to me that SQL pass-through is kind of contrary to what the OP needs.
A query in SQL pass-through mode will be executed as if it was typed in psql, that's the point of this mode. So refering to an Access table like your example query does is not going to work.
What the OP can do is attach the CSV file in Access so that it will appear as a table and should be usable as such in an update or insert query within Access query builder. It's quite straightforward, there's no need for temp files or COPY.
Best regards,
TO quote the OP "I’m using Access as that is a point of comfort for me. I can write a .net procedure though I’m not exactly firm on the procedure."
So i thought it would be nice to point the OP in the right direction in getting that to work. learning how to setup connection strings and write SQL queries in access will carry over to pretty much everything else.