It would be really helpful to be able to reposition columns in tables. That way, one could: add the new TIMESTAMP column, populate it using to_timestamp(), drop the text column, reposition the TIMESTAMP column to where it "should" be.
If that would give you what you need then just define the column as text initially, load the data, then do an ALTER COLUMN ... ALTER TYPE to change the column type to timestamptz in place, with the conversion done via USING.