This link:

shows that apache allows files to be included in it's configure files, but it did not work when I tried it in .htaccess. Does anyone know if it's supposed to?

In one site I put this in .htaccess:
Include /home/test/public_html/dynamic/
in the test account's public_html/dynamic directory, I put a file with this content:
ErrorDocument 404 /home/test/public_html/404_error.php
I verified that 404_error.php exists in the specified path.