Nahradit proměnné MySQL v PostgreSQL?

Použijte VALUES() v SELECT, to by mělo fungovat:

INSERT INTO employees (
    company_id,
    name,
    position,
    created_by,
    last_modified_by
)
SELECT
    (SELECT id FROM companies WHERE name = 'Acme Fellowship'),
    name,
    position,
    (SELECT id FROM users WHERE login = 'admin'),
    (SELECT id FROM users WHERE login = 'admin')
FROM
    (VALUES -- all your new content here
        ('Frodo Baggins',  'Ring bearer'), 
        ('Samwise Gamgee', 'Rope bearer'), 
        ('Peregrin Took',  'Ent rider')
    ) content(name, position); -- use some aliases to make it readable