first of all check if your system is using crypt() or MD5 passwords (RedHat 6..7 uses MD5) if your system uses MD5 passwords then you have to encrypt them with something like that:

use Crypt::PasswdMD5;

sub my_md5_crypt {
my $salt;

for(0..7) {
    $salt .= ('.', '/', 0..9, 'A'..'Z','a'..'z')[rand 64];
}
my $pass = shift; #clear text password

return unix_md5_crypt($pass, '$1$' . $salt . '$');
}
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in any case you should build a file in this format:

USERNAME:ENCRYPTEDPASSWORD
USERNAME2:ENCRYPTEDPASSWORD2
...
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and then

cat THATFILE | /usr/sbin/chpasswd -e
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Note: if you have normal crypt passwords just build a file like that:

USERNAME1:CLEARTEXTPASS_1
...
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and then "/usr/sbin/chpasswd < THATFILE"