To add Cash Payment option, you need to create a module based on

http://docs.whmcs.com/Bank_Transfer

Create file

modules/gateways/cash.php

With following content

<?php

# Cash Payment Gateway Module

if (!defined("WHMCS")) die("This file cannot be accessed directly");

function cash_config() {

    $configarray = array(
     "FriendlyName" => array(
        "Type" => "System",
        "Value" => "Cash Payment"
        ),
     "instructions" => array(
        "FriendlyName" => "Cash Payments Instructions",
        "Type" => "textarea",
        "Rows" => "5",
        "Value" => "Bank Name:\nPayee Name:\nSort Code:\nAccount Number:",
        "Description" => "The instructions you want displaying to customers who choose this payment method - the invoice number will be shown underneath the text entered above",
        ),
    );

    return $configarray;

}

function cash_link($params) {
    global $_LANG;

    $code = '<p>'.nl2br($params['instructions']).'<br />'.$_LANG['invoicerefnum'].': '.$params['invoiceid'].'</p>';

    return $code;

}

To Activate plugin, go to

Setup > Payments > Payment Gateways

On next Page, you will see the new Cash Payment Gateway. Click on it to activate.

If you don’t show cash payment option during online order process, you can un-check the Show on Order Form check box.