You may provide the password already hashed with md5, as said in the doc (CREATE ROLE):
ENCRYPTED UNENCRYPTED These key words control whether the password is
stored encrypted in the system catalogs. (If neither is specified, the
default behavior is determined by the configuration parameter
password_encryption.) If the presented password string is already in
MD5-encrypted format, then it is stored encrypted as-is, regardless of
whether ENCRYPTED or UNENCRYPTED is specified (since the system cannot
decrypt the specified encrypted password string). This allows
reloading of encrypted passwords during dump/restore.
The information that’s missing here is that the MD5-encrypted string should be the password concatened with the username, plus md5 at the beginning.
So for example to create u0 with the password foobar, knowing that md5('foobaru0') is ac4bbe016b808c3c0b816981f240dcae:
CREATE USER u0 PASSWORD 'md5ac4bbe016b808c3c0b816981f240dcae';
and then u0 will be able to log in by typing foobar as the password.
I don’t think that there’s currently a way to use SHA-256 instead of md5 for PostgreSQL passwords.