Are you intending to use client software to access the source data or are you planning on putting the source data in a location where the server o/s user can see it?
If you already have an actual temporary table inside the database you wouldn't use COPY. COPY is intended to transfer data from/to an external file (including stdin/stdout).
Generally:
(in psql)
BEGIN;
CREATE TEMP TABLE tmptbl ( cols );
\copy tmptbl from '/tmp/file-to-load.csv' with ( ... )
INSERT INTO tbl SELECT * FROM tmptbl;
COMMIT;
\copy in psql constructs an appropriate "COPY" SQL command, executes it on the server, and then funnels the contents of "file" to the server.
